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

R=thibaudm@chromium.org

Bug: v8:12887
Change-Id: Id2f457a1c0056d5015e2f9983d4599582d7189cd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3876185Reviewed-by: Thibaud Michaud <thibaudm@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83219}