optimize cond waiter move using atomic swap instead of cas loop

src/thread/pthread_cond_broadcast.c

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