rep stos appears to be much slower for small stacks. Use pushq instead,
in a loop, unrolled to push eight at a time.

Bug: v8:7700
Change-Id: Ieddf52ed514cb86f5c1dd7b159cc9a1a1c440103
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3702440Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Toon Verwaest <verwaest@chromium.org>
Auto-Submit: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#81140}