Restore some non-POSIX functions for AIX

[xingxue-ibm]

Description

These non-POSIX functions were previously removed because they are guarded by the _KERNEL macro in AIX headers. After discussion, we concluded they might be useful, so this patch restores them with necessary fixes.

Sources

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]

Unknown labels: O-unix, S-waiting-on-review