When freezing flags, not only remember this in a global variable, but
also actually memory-protect the memory that holds the flag values.

R=cbruni@chromium.org
CC=​sroettger@chromium.org

Bug: v8:12887
Cq-Include-Trybots: luci.v8.try:v8_linux_blink_rel
Change-Id: I2ae638790d1f08f4bcc1b7e6cb5970e4e7463aad
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3811286Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82478}