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

R=mliedtke@chromium.org

Bug: v8:12887
Change-Id: I417eee6311fadef9b60043cfc9a42926859c7ab9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3899304Reviewed-by: Matthias Liedtke <mliedtke@chromium.org>
Commit-Queue: Matthias Liedtke <mliedtke@chromium.org>
Cr-Commit-Position: refs/heads/main@{#83247}