spinlock/fas: improve codegen for the uncontended path

Mark acquisition failures as unlikely: there's no point in reaching
the spinloop slow path more quickly.

TESTED=existing regression tests.

include/spinlock/fas.h

@@ -77,9 +77,10 @@ CK_CC_INLINE static void
 ck_spinlock_fas_lock(struct ck_spinlock_fas *lock)
 {
 
-	while (ck_pr_fas_uint(&lock->value, true) == true) {
-		while (ck_pr_load_uint(&lock->value) == true)
+        while (CK_CC_UNLIKELY(ck_pr_fas_uint(&lock->value, true) == true)) {
+                do {
                         ck_pr_stall();
+                } while (ck_pr_load_uint(&lock->value) == true);
         }
 
 	ck_pr_fence_lock();