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

R=jkummerow@chromium.org

Bug: v8:12887
Change-Id: I5bd5faaac89185c5f40b0eabb01f9b678f791498
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3898934
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83225}