Use a non-locking initial test in TAS_SPIN on PPC.

Further testing convinces me that this is helpful at sufficiently high
contention levels, though it's still worrisome that it loses slightly
at lower contention levels.

Per Manabu Ori.

src/include/storage/s_lock.h

@@ -358,6 +358,9 @@ typedef unsigned int slock_t;
 
 #define TAS(lock) tas(lock)
 
+/* On PPC, it's a win to use a non-locking test before the lwarx */
+#define TAS_SPIN(lock)	(*(lock) ? 1 : TAS(lock))
+
 /*
  * NOTE: per the Enhanced PowerPC Architecture manual, v1.0 dated 7-May-2002,
  * an isync is a sufficient synchronization barrier after a lwarx/stwcx loop.