htl: Make sure the exit path of last thread sees all thread cleanups

In case e.g. some atexit() handlers expect all threads to have finished
their side effects.

Reported-by: Brent Baccala <cosine@freesoft.org> 's Claude assistant

htl/pt-exit.c

@@ -60,7 +60,7 @@ __pthread_exit (void *status)
 
   /* Decrease the number of threads.  We use an atomic operation to
      make sure that only the last thread calls `exit'.  */
-  if (atomic_fetch_add_relaxed (&__pthread_total, -1) == 1)
+  if (atomic_fetch_add_acq_rel (&__pthread_total, -1) == 1)
     /* We are the last thread.  */
     exit (0);