The new pagezero_size introduced by CL 72730 breaks
on 32-bit systems, since it is 2³². Restrict the change to
darwin/arm64, since it is intended for iOS only.

We could plausibly allow GOARCH=amd64 as well, but
without a compelling reason, changing the zero page size
doesn't seem worth the risk.

Change-Id: I5d6adcbaff8d0e5b169ff13512f188332cc7ed9a
Reviewed-on: https://go-review.googlesource.com/73250
Run-TryBot: Russ Cox <rsc@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>