OS::Allocate() previously was calling zx_vmo_replace_as_executable()
when executable access is requested. This breaks the case when the
OS::SetPermission() is called to mark that memory as executable later.
Updated it to call zx_vmo_replace_as_executable() for all VMOs.

This solution is not ideal, but it's consistent with other platforms,
so it's acceptable short-term.

Bug: v8:8899, chromium:934582
Change-Id: Ifeb818c93d8b9c80e73a057f5e6f3ca5a7b1c23f
Reviewed-on: https://chromium-review.googlesource.com/c/1483613Reviewed-by: Wez <wez@chromium.org>
Commit-Queue: Sergey Ulanov <sergeyu@chromium.org>
Cr-Commit-Position: refs/heads/master@{#59880}