https://crrev.com/c/3293410 added concurrent processing of C++ objects
found through V8 embedder fields. The CL missed that those embedder
fields are not read atomically from JS objects. The problem is that
embedder fields are only aligned to kTaggedSize on builds with pointer
compression and are as such mis-aligned for atomic ops. This is not a
problem for on-heap values as the upper 32bits are anyways computed
from the cage. Is is a problem for generic C++ values though, as they
are used with Oilpan.

This CL adds the standard marker snapshot protocol for embedder fields.

Marker:
1. Snapshot embedder fields
2. Try to mark host object
3. On success: process snapshot

Main thread:
1. On setting embedder fields mark the object black first
2. Emit a write barrier for the embedder fields

This will get simpler with the heap sandbox that uses a separate table
for embedder fields. Once the sandbox is the default configuration, we
	can use it as dependency for the concurrent fast path.

Bug: chromium:1285706
Change-Id: I6b975ea561be08cda840ef0dd27a11627de93900
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3380983Reviewed-by: Dominik Inführ <dinfuehr@chromium.org>
Commit-Queue: Michael Lippautz <mlippautz@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78604}