With the smi-corrupting decompression approach we don't have to sign
extend Smis anymore and therefore we can switch to zero extending
approach by moving the isolate root to the beginning of the reserved
4Gb region.

Bug: v8:9706
Change-Id: Icd6008fa87d0924519b574fdec445976f742e306
Reviewed-on: https://chromium-review.googlesource.com/c/v8/v8/+/1835548
Commit-Queue: Igor Sheludko <ishell@chromium.org>
Reviewed-by: Jakob Gruber <jgruber@chromium.org>
Reviewed-by: Toon Verwaest <verwaest@chromium.org>
Cr-Commit-Position: refs/heads/master@{#64144}