- Solves a problem for PPC in a configuration where commit page size
  is 64K. https://chromium-review.googlesource.com/c/v8/v8/+/1149515
- Uses existing VM allocation code to get properly aligned memory.
- Makes sure the size for SetPermissions is a multiple of system page
  size.

Bug:chromium:756050

Change-Id: Ib3799ab7a3bb44b0091c234234c1cc47938379c2
Reviewed-on: https://chromium-review.googlesource.com/1161210
Commit-Queue: Bill Budge <bbudge@chromium.org>
Reviewed-by: Michael Lippautz <mlippautz@chromium.org>
Reviewed-by: Michael Starzinger <mstarzinger@chromium.org>
Cr-Commit-Position: refs/heads/master@{#54930}