v8_enable_v8_checks has very little coverage outside of V8 itself.
Move pointer verification checks behind DEBUG so that they fire in
regular debug or dcheck_always_on builds.

Change-Id: Ib2803240dd996f4223e403d20e927aff2955afbc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3242006Reviewed-by: Omer Katz <omerkatz@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#77534}