rts: do not prefetch mark_closure bdescr in non-moving gc when ASSERTS_ENABLED

This commit fixes a small an oversight in !12148: the prefetch logic
in non-moving GC may trap in debug RTS because it calls Bdescr() for
mark_closure which may be a static one. It's fine in non-debug RTS
because even invalid bdescr addresses are prefetched, they will not
cause segfaults, so this commit implements the most straightforward
fix: don't prefetch mark_closure bdescr when assertions are enabled.

(cherry picked from commit 886ab43a)

rts/sm/NonMovingMark.c

@@ -919,7 +919,9 @@ static MarkQueueEnt markQueuePop (MarkQueue *q)
         // MarkQueueEnt encoding always places the pointer to the object to be
         // marked first.
         prefetchForRead(&new.mark_closure.p->header.info);
+#if !defined(ASSERTS_ENABLED)
         prefetchForRead(Bdescr((StgPtr) new.mark_closure.p));
+#endif
         q->prefetch_queue[i] = new;
         i = (i + 1) % MARK_PREFETCH_QUEUE_DEPTH;
     }