x86: provide build time option to support up to 123Tb of memory

x86: provide build time option to support up to 123Tb of memory

As this requires growing struct page_info from 32 to 48 bytes as well
as shrinking the always accessible direct mapped memory range from 5Tb
to 3.5Tb, this isn't being introduced as a general or default enabled
feature.

A side effect of the change to x86's mm.h is that asm/mm.h may no
longer be included directly. Hence in the few places where this was done,
xen/mm.h is being substituted (indirectly in the hvm/mtrr.h case).

Signed-off-by: Jan Beulich <jbeulich@suse.com>
Reviewed-by: Tim Deegan <tim@xen.org>
Reviewed-by: Andrew Cooper <andrew.cooper3@citrix.com>