fix read-after-free type error in pthread_detach

calling __unlock on t->exitlock is not valid because __unlock reads
the waiters count after making the atomic store that could allow
pthread_exit to continue and unmap the thread's stack and the object t
points to. for now, inline the __unlock logic with an unconditional
futex wake operation so that the waiters count is not needed.

once __lock/__unlock have been made safe for self-synchronized
destruction, we could switch back to using them.

src/thread/pthread_detach.c

@@ -9,7 +9,8 @@ static int __pthread_detach(pthread_t t)
 	if (a_swap(t->exitlock, 1))
 		return __pthread_join(t, 0);
 	t->detached = 2;
-	__unlock(t->exitlock);
+	a_store(t->exitlock, 0);
+	__wake(t->exitlock, 1, 1);
 	return 0;
 }