Support for running on non-Unix hosts

[bjorn3]

There are a bunch of places where Unix-only apis are used. While Wild is currently an ELF-only linker and thus mostly usable for Unix targets (though there are a bunch of non-Unix OSes that use ELF), it would still be useful to use Wild on non-Unix hosts when cross-compiling from Windows. Or in my case compiling Wild to WASI. Would changes to support this be accepted? When compiling for WASI, I saw the following issues:

• Compilation fails on WASI even when the enabled feature flag is disabled davidlattimore/perfetto-recorder#9 (not relevant for Windows, fix merged)
• Bunch of std::os::unix::ffi::OsStrExt uses. (feat: Support building on Windows #1629)
• default_target_arch doesn't compile on wasm32.
• Rayon fails to initialize on WASI unless .use_current_thread() is used.
• O_CLOEXEC (revert #548 - it's unnecessary #1463)
• make_executable (can be a no-op outside of Unix hosts)
• std::os::unix::fs::symlink
• fork (The Windows kernel technically supports fork, but it is even slower than spawning a new process, which is already slow. Probably no benefit to gain from using the fork trick on Windows) (feat: Support building on Windows #1629)

[davidlattimore]

That all sounds good to me. O_CLOEXEC is #1463

I'll be interested to see what the implications of .use_current_thread are. So long as we can have all CPUs busy with one thread each, I'm happy.

There's #1407 that was aiming to build on windows. It hasn't seen activity for a few weeks though, but if you wanted, I could push changes to their branch and then merge it.

Once it's all done, we should add CI jobs to verify that we don't break it.

[bjorn3]

Wasi doesn't have threads at all right now, so .use_current_thread() is used to disable usage of threads. Something that works on wasi and I think wouldn't affect performance would be to use that option when the thread pool would have a single thread. The jobserver crate doesn't return any tokens on wasi, so the implicit token results in a single worker thread being requested by default on wasi.

[fahdfady]

I'm interested in working on this. not sure that i'll complete the whole checklist but i'll probably be starting with the std::os::unix usage within the codebase and see what i could do :)