Moves g_root_vmar_base up in the file, so that we have all
the globals together.

Bug: v8:11232
Change-Id: Ic08cdf3399982962de255028be6718951a17aedb
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/3416249
Auto-Submit: Victor Gomes <victorgomes@chromium.org>
Reviewed-by: Samuel Groß <saelo@chromium.org>
Commit-Queue: Samuel Groß <saelo@chromium.org>
Cr-Commit-Position: refs/heads/main@{#78786}