AllocateGuarded previously fell back on Allocate and then called Guard
to set the protection to PROT_NONE. Linux commits RW memory, but the
important thing here is to reserve the address space without committing
it. This change adds a new variant of Allocate that takes explicit
permission bits so that AllocateGuarded allocates non-RW memory from the
beginning.

Bug: v8:6320
Change-Id: I7962acbed09938951bf3bb4af2d1f302adba2547
Reviewed-on: https://chromium-review.googlesource.com/491928
Commit-Queue: Eric Holk <eholk@chromium.org>
Reviewed-by: Mircea Trofin <mtrofin@chromium.org>
Reviewed-by: Jochen Eisinger <jochen@chromium.org>
Cr-Commit-Position: refs/heads/master@{#45075}