[fastcall] Simplify JS execution flag setting

This CL improves setting the javascript_execution_assert
on the isolate from generated code by adding a single wrapper
function around the fast call creation, along with its setup
and teardown. It's a follow-up for
https://chromium-review.googlesource.com/c/v8/v8/+/2982998/

Bug: chromium:1218898
Change-Id: I0dc45520aa2a9b60ce03076a495e3f68c05945c7
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2999086


Reviewed-by: Georg Neis <neis@chromium.org>
Commit-Queue: Maya Lekova <mslekova@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75506}