Run-time feature detection for Aarch64 on Windows.

[makotokato]

Windows has API to detect CPU features for crc32 and ARM crypto extension.

[rust-highfive]

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @gnzlbg (or someone else) soon.

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[gnzlbg]

Suggested change

	[target.'cfg(target_os="windows")'.dependencies]
	[target.'cfg(all(target_os = "windows", target_arch = "aarch64"))'.dependencies]

[gnzlbg]

Beyond the Cargo.toml nitpick this looks fine to me.

I don't know how complex the implementation of the FFI wrappers for this API is in winapi, but it might speed things up to remove the winapi dependency for now, and re-implementing it ourselves here.

We can then remove that and move towards using winapi in libstd as part of a future refactoring, that only needs to deal with the "organizational" issues of adding a new dependency to the Rust language runtime.

EDIT: the winapi wrapper without the definitions looks quite simple, and we only need to make sure this works for the aarch64 target, so it might be worth it to just copy paste what we need: https://github.com/retep998/winapi-rs/blob/a6dd5510212a2f0f5aae66e60949bd6fdaa0a44d/src/um/processthreadsapi.rs#L337

[makotokato]

If I shouldn't use winapi, I remove this, then I add FFI to call Win32 API without winapi.

[gnzlbg]

If I shouldn't use winapi, I remove this, then I add FFI to call Win32 API without winapi.

I think this feature is important, and removing winapi would definitely help landing this faster. After this has landed on rust-lang/rust, you can submit another PR that changes this to use winapi, and we can slowly work out through all organizational issues - that would usually take on the order of weeks if an FCP turns out to be required.

[gnzlbg]

Modulo nitpicks, this looks great!

[gnzlbg]

Thank you!