This adds a few DCHECKs to ensure that the process-wide memory
protection key is not writable (per thread) in a few strategic places:
- Before switching it to writable (which implicitly checks the initial
    state),
- when entering compiled code, and
- in the explicit unit test.

R=jkummerow@chromium.org
CC=mpdenton@chromium.org

Bug: v8:11974
Change-Id: I6037f599afe9009d5e48794eb382eb1979f3ce9f
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3165060Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#76953}