The Schönhage-Strassen method for *very* large inputs.

This is a reland of 347ba357,
with added zero-initialization to pacify MSan (spurious report).

Originally:
> Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3000742
> Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
> Reviewed-by: Maya Lekova <mslekova@chromium.org>
> Cr-Commit-Position: refs/heads/master@{#75659}

Bug: v8:11515
Change-Id: Ieac6e174bde6eb09af0a9a9a49969feabca79e81
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3018081Reviewed-by: Maya Lekova <mslekova@chromium.org>
Commit-Queue: Jakob Kummerow <jkummerow@chromium.org>
Cr-Commit-Position: refs/heads/master@{#75663}