x86: mmu: add runtime modification indicator

The CPU first checks the page directory entry for write
or user permissions on a particular page before looking
at the page table entry.

If a region configured all pages to be non user accessible,
and this was changed for a page within it to be accessible,
the PDE would not be updated and any access would still
return a page fault.

The least amount of runtime logic to deal with this is to
indicate at build time that some pages within a region may
be marked writable or user accessible at runtime, and to
pre-set the flags in the page directory entry accordingly.

The driving need for this is the region configuration for
kernel memory, which will have user permissions set at
runtime for stacks and user-configured memory domains.

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