This change makes full 8 GiB guard regions always enabled on 64-bit
platforms.

Additionally, since all Wasm memory allocation paths have some form of
guard regions, this removes and simplifies most of the logic around
whether to enable guard regions.

R=gdeepti@chromium.org

Change-Id: Idf3fbcc11ac70ea2ee7eb88c2173d6a1410395e1
Reviewed-on: https://chromium-review.googlesource.com/985142
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Brad Nelson <bradnelson@chromium.org>
Cr-Commit-Position: refs/heads/master@{#52310}