In backing-store.cc, define GetGuardedRegion(), which is only used on
64-bit, only when V8_TARGET_ARCH_64_BIT evals to true. Then add
GetRegion(), which returns the appropriate region depending on whether
guards are enabled or not, and use it to simplify the only caller to
GetGuardedRegion().

Similarly, define |kFullGuardSize| as 64-bit only, and add
GetReservationSize() to only access |kFullGuardSize| when appropriate on
64-bit platforms.

Change-Id: Iefae7969a6138118d466a9d48e0ea62d94ff07f3
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1899547
Commit-Queue: Lei Zhang <thestig@chromium.org>
Reviewed-by: Ulan Degenbaev <ulan@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64782}