Add wait4 for macOS and FreeBSD

[darcyliu]

This PR aims to add the system call wait4 which is supported by macOS and FreeBSD.

[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.

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 Due to the way GitHub handles out-of-date commits, this should also make it reasonably obvious what issues have or haven't been addressed.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.

[alexcrichton]

Thanks for the PR here, but right now there's not many maintainers to help merge this, so it may take awhile.

[JohnTitor]

@darcyliu Could you rebase to check current CI status?
r? @JohnTitor

[darcyliu]

Hi @JohnTitor I've updated.

[JohnTitor]

Looks good! Could you also squash commits into one to make log cleaner?

[JohnTitor]

Thanks!