rts: Use fence rather than redundant load

Previously we would use an atomic load to ensure acquire ordering.
However, we now have `ACQUIRE_FENCE_ON`, which allows us to express this
more directly.

rts/sm/GCAux.c

@@ -91,7 +91,7 @@ isAlive(StgClosure *p)
         return TAG_CLOSURE(tag,(StgClosure*)UN_FORWARDING_PTR(info));
     }
 
-    info = ACQUIRE_LOAD(&q->header.info);
+    ACQUIRE_FENCE_ON(&q->header.info);
     info = INFO_PTR_TO_STRUCT(info);
 
     switch (info->type) {