fix more instances of old a_xchg (use new a_swap name)

src/thread/pthread_detach.c

@@ -3,7 +3,7 @@
 int pthread_detach(pthread_t t)
 {
 	/* Cannot detach a thread that's already exiting */
-	if (a_xchg(&t->exitlock, 1))
+	if (a_swap(&t->exitlock, 1))
 		return pthread_join(t, 0);
 	t->detached = 1;
 	t->exitlock = 0;

src/thread/pthread_spin_lock.c

@@ -2,6 +2,6 @@
 
 int pthread_spin_lock(pthread_spinlock_t *s)
 {
-	while (a_xchg(s, 1)) a_spin();
+	while (a_swap(s, 1)) a_spin();
 	return 0;
 }

src/thread/pthread_spin_trylock.c

@@ -2,5 +2,5 @@
 
 int pthread_spin_trylock(pthread_spinlock_t *s)
 {
-	return -a_xchg(s, 1) & EBUSY;
+	return -a_swap(s, 1) & EBUSY;
 }