Bug 1670885 - Fix post-fork() handlers for PHC/LogAlloc to work on macOS using...

Bug 1670885 - Fix post-fork() handlers for PHC/LogAlloc to work on macOS using unfair locks r=glandium

macOS unfair locks enforce that a lock can only be released by the thread which locked it.

On macOS 11+ this caused the fork()'d child process to raise a SIGILL signal. Confusingly enough this behavior seems to be different on macOS 10.15 and possibly interacted in odd ways with our exception handler if it was installed before fork()-ing.

Differential Revision: https://phabricator.services.mozilla.com/D148287