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

R=ishell@chromium.org

Bug: v8:12887
Change-Id: Ic54ad0c1135f6d3ff9f261df40f2138bf511d5be
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3876383Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83013}