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

R=ishell@chromium.org

Bug: v8:12887
Change-Id: I2ef25bc50fdf12f0149f2cdfce7102f2cc0f25d1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899196Reviewed-by: Igor Sheludko <ishell@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83261}