condition variable signal/bcast need not wake unless there are waiters

src/thread/pthread_cond_broadcast.c

@@ -2,7 +2,7 @@
 
 int pthread_cond_broadcast(pthread_cond_t *c)
 {
-	c->_c_block = 0;
+	if (a_swap(&c->_c_block, 0))
 		__wake(&c->_c_block, -1, 0);
 	return 0;
 }

src/thread/pthread_cond_signal.c

@@ -2,7 +2,7 @@
 
 int pthread_cond_signal(pthread_cond_t *c)
 {
-	c->_c_block = 0;
+	if (a_swap(&c->_c_block, 0));
 		__wake(&c->_c_block, 1, 0);
 	return 0;
 }