optimize contended case for pthread_spin_trylock

src/thread/pthread_spin_trylock.c

@@ -2,5 +2,6 @@
 
 int pthread_spin_trylock(pthread_spinlock_t *s)
 {
-	return -a_xchg(s, 1) & EBUSY;
+	if (*s || a_xchg(s, 1)) return EBUSY;
+	return 0;
 }