Avoid the deprecated FLAG_* syntax, access flag values via the
{v8_flags} struct instead.

R=nicohartmann@chromium.org

Bug: v8:12887
Change-Id: Icc9016552c53489afd821ff6d360c025e48fee18
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3871201
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Nico Hartmann <nicohartmann@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82981}