In this CL, Code object layout changes s.t. the instruction
area is distinct / non-overlapping from the metadata area.

On-heap Code objects now have a variable-size `body` area,
containing distinct-but-adjacent `instruction` and `metadata`
areas.

Off-heap code (= embedded builtins) currently have the same,
but in the future the metadata area will move elsewhere and
no longer be adjacent to instructions.

To implement this, the main changes are:

 - The Code object header now contains instruction and metadata
   sizes, and no longer contains the safepoint table offset
   (it's implicitly the first table of the metadata section).
 - The embedded metadata table contains information about both
   instruction and metadata areas.

I've also added assertions in spots that currently rely on a
contiguous body area.

Bug: v8:11036
Change-Id: I940f0c70c07ad511dafd2d2c3e337de8c92cd4b9
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491025Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70743}