1. Remove {kOneGiB}, use the global {GB} instead.
2. Inline {GetGuardedRegion} into the single caller {GetRegion}, to
   remove another "#if V8_TARGET_ARCH_64_BIT".
3. Rename {GetRegion} to {GetReservedRegion}.

R=ahaas@chromium.org

Bug: v8:10933
Change-Id: I3b26ae0036db0d910165ed4d993d27262f2cb5ae
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/2491035Reviewed-by: Andreas Haas <ahaas@chromium.org>
Commit-Queue: Clemens Backes <clemensb@chromium.org>
Cr-Commit-Position: refs/heads/master@{#70734}