To try and reduce StringBuilder's dependencies, use std::memcpy instead
of the V8-only MemCopy.

Change-Id: I576dccd4a2ff1b796314f8e806cbb0c70f6c07f2
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2972730
Commit-Queue: Dan Elphick <delphick@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Auto-Submit: Dan Elphick <delphick@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75244}