[nojit] Don't flush the icache in jitless mode

We don't allocate executable memory in jitless mode hence there's no
need to flush the icache.

Bug: v8:7777
Change-Id: I70a1884e6c9f11405465f5741f2eccd4f7a273fb
Reviewed-on: https://chromium-review.googlesource.com/c/1488765
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59887}

src/flush-instruction-cache.cc

@@ -13,6 +13,7 @@ namespace internal {
 
 void FlushInstructionCache(void* start, size_t size) {
   if (size == 0) return;
+  if (FLAG_jitless) return;  // There's no executable memory to flush.
 
 #if defined(USE_SIMULATOR)
   base::MutexGuard lock_guard(Simulator::i_cache_mutex());