libredirect: Add preload wrapper for stat()

Pull request #50246 was merged a bit too quickly and it was supposed to
fix libredirect on Darwin. However it still failed on Darwin and this
was missed by the person merging the pull request.

The reason this was failing was that there is no __xstat* on Darwin.

So I'm adding a wrapper for stat() as well as it works on Darwin and it
still doesn't hurt on GNU/Linux.

Signed-off-by: aszlig <aszlig@nix.build>
Cc: @edolstra, @zimbatm