Fix broken #ifdef for __sparcv8

Rob Rowan. Backpatch to all supported versions, like the patch that added
the broken #ifdef.

src/include/storage/s_lock.h

@@ -434,7 +434,7 @@ tas(volatile slock_t *lock)
  * requires a barrier.
  */
 #define S_UNLOCK(lock)		(*((volatile slock_t *) (lock)) = 0)
-#elif  __sparcv8
+#elif defined(__sparcv8)
 /* stbar is available (and required for both PSO, RMO), membar isn't */
 #define S_UNLOCK(lock)	\
 do \