Port `run-make/sysroot-crates-are-unstable` to rmake

I already have a more elaborate draft at rust-lang#126231 that tries to port the underlying Python script to rmake, but there's no need for the removal of Makefiles to be held up on complex tasks like that, so this PR simply takes the trivial Makefile and converts it into a trivial rmake recipe.

Part of rust-lang#121876.

r? `@jieyouxu`

Port `run-make/sysroot-crates-are-unstable` to rmake

I already have a more elaborate draft at rust-lang#126231 that tries to port the underlying Python script to rmake, but there's no need for the removal of Makefiles to be held up on complex tasks like that, so this PR simply takes the trivial Makefile and converts it into a trivial rmake recipe.

Part of rust-lang#121876.

r? ``@jieyouxu``

Rollup merge of rust-lang#129071 - Zalathar:sysroot-unstable, r=jieyouxu

Port `run-make/sysroot-crates-are-unstable` to rmake

I already have a more elaborate draft at rust-lang#126231 that tries to port the underlying Python script to rmake, but there's no need for the removal of Makefiles to be held up on complex tasks like that, so this PR simply takes the trivial Makefile and converts it into a trivial rmake recipe.

Part of rust-lang#121876.

r? ``@jieyouxu``

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? `@jieyouxu`

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? `@jieyouxu`

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? `@jieyouxu`

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? ``@jieyouxu``

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? ```@jieyouxu```

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? ````@jieyouxu````

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw

Rollup merge of rust-lang#129111 - Zalathar:python-sysroot, r=jieyouxu

Port the `sysroot-crates-are-unstable` Python script to rmake

New version of rust-lang#126231, and a follow-up to rust-lang#129071.

One major difference is that the new version no longer tries to report *all* accidentally-stable crates, because the `run_make_support` helpers tend to halt the test as soon as something goes wrong. That's unfortunate, but I think it won't matter much in practice, and preserving the old behaviour doesn't seem worth the extra effort.

---

Part of rust-lang#110479 (Python purge), with this being one of the non-trivial Python scripts that actually seems feasible and worthwhile to remove.

This is *not* part of rust-lang#121876 (Makefile purge), because the underlying test is already using rmake; this PR just modifies the existing rmake recipe to do all the work itself instead of delegating to Python. So there's no particular urgency here.

r? ````@jieyouxu````

try-job: aarch64-gnu
try-job: aarch64-apple
try-job: test-various
try-job: armhf-gnu
try-job: x86_64-msvc
try-job: i686-mingw