In threaded processes, destroy the mutex atexit_mutex when we've

In threaded processes, destroy the mutex atexit_mutex when we've
finished using it.  This allows the mutex's allocated memory to be
freed.

This is one sense a rather silly change, since at this point we're
less than a microsecond away from calling _exit; but fixing this
memory leak is likely to make life easier for anyone trying to
track down other memory leaks.