Add more Cygwin stuff

[500-internal-server-error]

Description

These constants and functions are required for building uutils-coreutils for Cygwin.

I'm not sure what the logic is on where things are placed, I just made it work. Additionally, I can't figure out how the tests work to fix it, but it's functional enough to unblock my immediate needs. Hopefully someone can give me guidance on how to clean up the changes and fix the tests?

Sources

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/cygwin/config.h#L50
https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/stdio.h#L138-L139

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/cygwin/utmp.h#L20-L33

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L21-L32

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/stdio.h#L120-L122

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/newlib/libc/include/grp.h#L64-L65

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L21-L32

https://github.com/msys2/msys2-runtime/blob/msys2-3.6.5/winsup/cygwin/include/utmpx.h#L42-L49

Checklist

• Relevant tests in libc-test/semver have been updated
• No placeholder or unstable values like *LAST or *MAX are
  included (see #3131)
• Tested locally (cd libc-test && cargo test --target mytarget);
  especially relevant for platforms that may not be checked in CI

@rustbot label +stable-nominated

[500-internal-server-error]

I don't think I touched MacOS, so I'm going to assume the fail is not due to my changes. I don't have hardware to test locally either...