Drive-by: Refactor FlushInstructionCache to its own header. This removes
dependencies of objects.cc and code.cc

Bug: v8:8562
Change-Id: If23f3b9d4f2068e08c61c0f4b070ecfe1b9a6cc0
Reviewed-on: https://chromium-review.googlesource.com/c/1456081Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Sigurd Schneider <sigurds@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59435}