revert previous change in cond var waiter move

using swap has a race condition: the waiters must be added to the
mutex waiter count *before* they are taken off the cond var waiter
count, or wake events can be lost.

src/thread/pthread_cond_broadcast.c

@@ -30,8 +30,12 @@ int pthread_cond_broadcast(pthread_cond_t *c)
 	}
 
 	/* Move waiter count to the mutex */
-	w = a_swap(&c->_c_waiters, 0);
-	a_fetch_add(&m->_m_waiters, w);
+	for (;;) {
+		w = c->_c_waiters;
+		a_fetch_add(&m->_m_waiters, w);
+		if (a_cas(&c->_c_waiters, w, 0) == w) break;
+		a_fetch_add(&m->_m_waiters, -w);
+	}
 
 	/* Perform the futex requeue, waking one waiter unless we know
 	 * that the calling thread holds the mutex. */