Improve spinlock performance for HP-UX, ia64, non-gcc.

At least on this architecture, it's very important to spin on a
non-atomic instruction and only retry the atomic once it appears
that it will succeed.  To fix this, split TAS() into two macros:
TAS(), for trying to grab the lock the first time, and TAS_SPIN(),
for spinning until we get it.  TAS_SPIN() defaults to same as TAS(),
but we can override it when we know there's a better way.

It's likely that some of the other cases in s_lock.h require
similar treatment, but this is the only one we've got conclusive
evidence for at present.

src/backend/storage/lmgr/s_lock.c

@@ -96,7 +96,7 @@ s_lock(volatile slock_t *lock, const char *file, int line)
 	int			delays = 0;
 	int			cur_delay = 0;
 
-	while (TAS(lock))
+	while (TAS_SPIN(lock))
 	{
 		/* CPU-specific delay each time through the loop */
 		SPIN_DELAY();