…r=cuviper

std support for wasm32 panic=unwind

Tracking issue: rust-lang#118168

This adds std support for `-Cpanic=unwind` on wasm, and with it slightly more fleshed out rustc support. Now, the stable default is still panic=abort without exception-handling, but if you `-Zbuild-std` with `RUSTFLAGS=-Cpanic=unwind`, you get wasm exception-handling try/catch blocks in the binary:

```rust
#[no_mangle]
pub fn foo_bar(x: bool) -> *mut u8 {
    let s = Box::<str>::from("hello");
    maybe_panic(x);
    Box::into_raw(s).cast()
}

#[inline(never)]
#[no_mangle]
fn maybe_panic(x: bool) {
    if x {
        panic!("AAAAA");
    }
}
```
```wat
;; snip...
(try $label$5
 (do
  (call $maybe_panic
   (local.get $0)
  )
  (br $label$1)
 )
 (catch_all
  (global.set $__stack_pointer
   (local.get $1)
  )
  (call $__rust_dealloc
   (local.get $2)
   (i32.const 5)
   (i32.const 1)
  )
  (rethrow $label$5)
 )
)
;; snip...
```

…kingjubilee

Rollup of 15 pull requests

Successful merges:

 - rust-lang#116791 (Allow codegen backends to opt-out of parallel codegen)
 - rust-lang#116793 (Allow targets to override default codegen backend)
 - rust-lang#117458 (LLVM Bitcode Linker: A self contained linker for nvptx and other targets)
 - rust-lang#119385 (Fix type resolution of associated const equality bounds (take 2))
 - rust-lang#121438 (std support for wasm32 panic=unwind)
 - rust-lang#121893 (Add tests (and a bit of cleanup) for interior mut handling in promotion and const-checking)
 - rust-lang#122080 (Clarity improvements to `DropTree`)
 - rust-lang#122152 (Improve diagnostics for parenthesized type arguments)
 - rust-lang#122166 (Remove the unused `field_remapping` field from `TypeLowering`)
 - rust-lang#122249 (interpret: do not call machine read hooks during validation)
 - rust-lang#122299 (Store backtrace for `must_produce_diag`)
 - rust-lang#122318 (Revision-related tweaks for next-solver tests)
 - rust-lang#122320 (Use ptradd for vtable indexing)
 - rust-lang#122328 (unix_sigpipe: Replace `inherit` with `sig_dfl` in syntax tests)
 - rust-lang#122330 (bootstrap readme: fix, improve, update)

r? `@ghost`
`@rustbot` modify labels: rollup

Rollup merge of rust-lang#121438 - coolreader18:wasm32-panic-unwind, r=cuviper

std support for wasm32 panic=unwind

Tracking issue: rust-lang#118168

This adds std support for `-Cpanic=unwind` on wasm, and with it slightly more fleshed out rustc support. Now, the stable default is still panic=abort without exception-handling, but if you `-Zbuild-std` with `RUSTFLAGS=-Cpanic=unwind`, you get wasm exception-handling try/catch blocks in the binary:

```rust
#[no_mangle]
pub fn foo_bar(x: bool) -> *mut u8 {
    let s = Box::<str>::from("hello");
    maybe_panic(x);
    Box::into_raw(s).cast()
}

#[inline(never)]
#[no_mangle]
fn maybe_panic(x: bool) {
    if x {
        panic!("AAAAA");
    }
}
```
```wat
;; snip...
(try $label$5
 (do
  (call $maybe_panic
   (local.get $0)
  )
  (br $label$1)
 )
 (catch_all
  (global.set $__stack_pointer
   (local.get $1)
  )
  (call $__rust_dealloc
   (local.get $2)
   (i32.const 5)
   (i32.const 1)
  )
  (rethrow $label$5)
 )
)
;; snip...
```

… r=bjorn3

Use throw intrinsic from stdarch in wasm libunwind

Tracking issue: rust-lang#118168

This is a very belated followup to rust-lang#121438; now that rust-lang/stdarch#1542 is merged, we can use the intrinsic exported from `core::arch` instead of defining it inline. I also cleaned up the cfgs a bit and added a more detailed comment.

… r=bjorn3

Use throw intrinsic from stdarch in wasm libunwind

Tracking issue: rust-lang#118168

This is a very belated followup to rust-lang#121438; now that rust-lang/stdarch#1542 is merged, we can use the intrinsic exported from `core::arch` instead of defining it inline. I also cleaned up the cfgs a bit and added a more detailed comment.

Rollup merge of rust-lang#131418 - coolreader18:wasm-exc-use-stdarch, r=bjorn3

Use throw intrinsic from stdarch in wasm libunwind

Tracking issue: rust-lang#118168

This is a very belated followup to rust-lang#121438; now that rust-lang/stdarch#1542 is merged, we can use the intrinsic exported from `core::arch` instead of defining it inline. I also cleaned up the cfgs a bit and added a more detailed comment.