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

R=nicohartmann@chromium.org

Bug: v8:12887
Change-Id: Ibdf60bd42ed577f367eee7da4de3a7e3dd6799e6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3871205Reviewed-by: Jakob Linke <jgruber@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83209}