New enum RelocInfo::COMPRESSED_EMBEDDED_OBJECT created to support
compressed pointers in generated code. Enum name EMBEDDED_OBJECT
changed to FULL_EMBEDDED_OBJECT.

RelocInfo::[set_]target_object() abstract away the difference between
FULL_EMBEDDED_OBJECT and COMPRESSED_EMBEDDED_OBJECT.

Compressed embedded objects can only be created at this time on
x64 with pointer compression turned on. Arm64 constant pools don't
support compressed objects at this time.

Bug: v8:7703
Change-Id: I03bfd84effa33c65cf9bcefa5df680ab7eace9dd
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1547661
Commit-Queue: Michael Stanton <mvstanton@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#61076}