optimize spin lock not to dirty cache line while spinning

src/thread/pthread_spin_lock.c

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