Add MADV_ZERO for macOS

[Thomasdezeeuw]

Description

Adds the MADV_ZERO constant for use in madvise(2).

Sources

• https://github.com/apple-oss-distributions/xnu/blob/f6217f891ac0bb64f3d375211650a4c1ff8ca1ea/bsd/sys/mman.h#L224

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

[tgross35]

LGTM but please squash

[JohnTitor]

Thanks!