Shrink RO_SPACE since it contains Immortal immovable objects and is
otherwise reporting a virtual size of 256KB when only half of that
will ever be used.

Bug: v8:9230, v8:7464
Change-Id: I68c17bb6c4ff12170774bad6a07dbb8b9d49cce1
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1906207
Commit-Queue: Dan Elphick <delphick@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64865}