Adds a new space RO_SPACE and modifies the serializer and other machinery
to support it.

Currently RO_SPACE has nothing in it, but will eventually contain all the
immovable immutable objects, so the GC can ignore it.

Bug: v8:7464
Cq-Include-Trybots: luci.chromium.try:linux_chromium_rel_ng
Change-Id: Ib2ff474699196c138df8c24f7a2248471e30fbac
Reviewed-on: https://chromium-review.googlesource.com/925703
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Hannes Payer <hpayer@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Yang Guo <yangguo@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52053}