Creates a single RelocInfo to be used by all builtin trampolines and
stores it as a root. All trampolines then substitute this for their
trampoline at generation time with DCHECKs to make sure it is
identical.

Also forces all non-trampoline RelocInfo ByteArrays for builtins to be
generated into RO_SPACE.

On x64, this results in the OLD_SPACE part of the startup snapshot
decreasing in size from 166096 to 131248 (-34848) bytes and RO_SPACE
(in the read-only snapshot) increasing from 31176 to 31248 (+72) bytes.

Bug: v8:8295
Change-Id: I69f4a899b738f2023ed42501c2b9797d34305b06
Reviewed-on: https://chromium-review.googlesource.com/c/1276468
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Cr-Commit-Position: refs/heads/master@{#56811}