https://github.com/abseil/abseil-cpp/issues/1518#issuecomment-1709098904
pointed out that the previous untested fix doesn't work because
pthread_getthreadid_np() has a different signature on Darwin.

Follow up to https://github.com/abseil/abseil-cpp/commit/b9707b7d7845f9710ae6d5906827b833fdcc2754

Fixes #1518

PiperOrigin-RevId: 563432451
Change-Id: Id0a9212e9c4413fa520a42934efaed2a06ca5dbc