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

R=jgruber@chromium.org

Bug: v8:12887
Change-Id: I0454426c664e54e9b8c8b39f903eeca1a80d4bc2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3898933
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Linke <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83214}