This CL makes the metadata offsets stored in Code headers
relative to the start of the metadata section (instead of to
the start of the instructions section).

In a follow-up, metadata for embedded builtins will be moved
from the .text section (with r-x or --x permissions) to the
.rodata section (with r-- permissions).

Drive-by: Simplify invariants around section alignment. A
new invariant is that the end of the instruction section is
aligned to Code::kMetadataAlignment.
Drive-by: Ensure trampoline Code objects contain no metadata
(metadata offsets all refer to the off-heap metadata section).

Tbr: dinfuehr@chromium.org
Bug: v8:11036
Change-Id: Idd0980913bbde9d3d1946b558e3ca58ec6356fcc
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491036Reviewed-by: Clemens Backes <clemensb@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Leszek Swirski <leszeks@chromium.org>
Commit-Queue: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70788}