Checks that flags1 are ReadOnly after SFI is finalised.

Bug: v8:12054
Change-Id: Ia2518b8f136a81aa076fd429bf4fcaf742a314e3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3263897
Commit-Queue: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77816}