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}