[gcc] Silence some more unused var warnings

Change-Id: Id3586eae6580416216bce2c448e6885aaa1589b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3732111


Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81430}