Update mcontext_t definition on musl, add fpstate struct

[MarkMcCaskey]

Hello! This PR updates definitions for x86_64 in musl to match the GNU ones and the current musl headers:

Relevant links for review:

• fpstate in musl header
• mcontext in musl header
• fpstate in Rust libc on GNU
• mcontext in Rust libc on GNU

[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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[MarkMcCaskey]

Hmm, I'm not sure why the tests are failing,

also the comment on mcontext on the musl side,

    // GitHub repo: ifduyue/musl/
    // commit: b4b1e10364c8737a632be61582e05a8d3acf5690
    // file: arch/x86_64/bits/signal.h#L80-L84

is probably out of date now. Is that comment being parsed and used for testing? 🤔

[bors]

☔ The latest upstream changes (presumably #1649) made this pull request unmergeable. Please resolve the merge conflicts.

[JohnTitor]

@MarkMcCaskey Could you rebase to check CI?
r? @JohnTitor

[JohnTitor]

I'm not sure if we could apply the same way as GNU since musl does a different way. At least, I think _libc_fpstate should be fpregset_t here.

[JohnTitor]

I'm going to close this as inactive. Feel free to re-open if you have some time to resolve the above comment, thanks!