Maglev is mid-tier optimising compiler designed mainly for compilation
speed that can still generate good code for straightforward JS.

This initial commit is an MVP for Maglev which can compile and run some
very simple code, and sets up a framework that we can build upon.

Design:
https://docs.google.com/document/d/13CwgSL4yawxuYg3iNlM-4ZPCB8RgJya6b8H_E2F-Aek/edit#

Bug: v8:7700
Change-Id: I5ae074ae099126c2c0d50864ac9b3d6fa5c9e85a
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3483664Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Commit-Queue: Leszek Swirski <leszeks@chromium.org>
Cr-Commit-Position: refs/heads/main@{#79247}