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

R=ishell@chromium.org

Bug: v8:12887
Change-Id: I457fd781f13c37ffdaa19e29c8f998ee3eaa55a5
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3875085Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#82996}