rustc doesn't handle relative symlinks to libraries

When rustc searches for libraries and encounters a symlink, instead of directly opening the link, it uses readlink and then uses the resulting path relative to cwd instead of relative to the link, causing it to fail to find the file.

The problem can be seen in the following excerpt from the output of strace on rustc:

...
[pid  2359] openat(AT_FDCWD, "/home/yuriks/projects/distrocast/.rust/lib/x86_64-unknown-linux-gnu", O_RDONLY|O_NONBLOCK|O_DIRECTORY|O_CLOEXEC) = 3
[pid  2359] getdents(3, /* 3 entries */, 32768) = 104
[pid  2359] getdents(3, /* 0 entries */, 32768) = 0
[pid  2359] close(3)                    = 0
[pid  2359] readlink("/home/yuriks/projects/distrocast/.rust/lib/x86_64-unknown-linux-gnu/librust-crypto-c53dba27-0.1.rlib", "../../../rust-crypto/librust-cry"..., 4096) = 53
[pid  2359] stat("../../../rust-crypto/librust-crypto-c53dba27-0.1.rlib", 0x7fd1ba017980) = -1 ENOENT (No such file or directory)
[pid  2359] write(2, "client.rs:5:1: 5:37 error: can't"..., 57client.rs:5:1: 5:37 error: can't find crate for `crypto`
...