Add UIO_MAXIOV constant to uclibc

[schultetwin1]

#1880 added UIO_MAXIOV to linux-like libc, but did not add it
to uclibc. This causes build breaks for std against uclibc.

[rust-highfive]

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

If any changes to this PR are deemed necessary, please add them as extra commits. This ensures that the reviewer can see what has changed since they last reviewed the code. Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed. Large or tricky changes may require several passes of review and changes.

Please see the contribution instructions for more information.

[JohnTitor]

Looks good, but CI didn't agree with it. Let's re-run it to see if it still happens.

[JohnTitor]

Found out that rust-lang/rust#74532 causes the failure on macOS nightly CI (s390x's is spurious and semver check's is unrelated to this). I'd skip the build check for that target if the upstream doesn't get a fix in a few days.

[schultetwin1]

Sounds good! Thanks for looking into that.

[JohnTitor]

Okay, it's now green. Thanks!

[schultetwin1]

Thank you!!