A minor improvement to copy over all bytes at once.

R=titzer@chromium.org
CC=binji@chromium.org

Change-Id: Ia9264a28afa76a5ed51d378d0db5bb192aeef2b6
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1511272
Commit-Queue: Clemens Hammacher <clemensh@chromium.org>
Reviewed-by: Ben Titzer <titzer@chromium.org>
Cr-Commit-Position: refs/heads/master@{#60121}