This allows to run a test repeatedly (in the same process) which
always sets the flag(s) to the same value. This also applies to
fuzzers.

The {FlagValue<T>::operator=} is the central bottleneck which is now
used for any flag value updates, either via the FLAG_foo globals, or
via the internal or public API.

R=cbruni@chromium.org

Bug: v8:12887, chromium:1346284
Change-Id: I46662322e1420ee12314544302ad9700523dcf90
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3780525
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Camillo Bruni <cbruni@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81909}