x86: use unused PTE bits when mapping memory

Page table management for x86 is being revised such that there
will not in many cases be a pristine, master set of page tables.
Instead, when mapping memory, use unused PTE bits to store the
original RW, US, and XD settings when the mapping was made.

This will allow memory domains to alter page tables while still
being able to restore the original mapping permissions.

Signed-off-by: Andrew Boie <andrew.p.boie@intel.com>