The regular x86 PTEs and nested PTEs are very similar but not identical.

In some instances they have bits that are equivalent but at different
positions. In other instances a bit in the regular PTE has no equivalent
in the nested PTE. The PG_M bit is an example of the former.

Undefine the PG_M macro in pmap.c so it is not inadvertently tested
with nested PTEs.

Use a function 'pmap_modified_bit(pmap_t)' to compute the bitmask for the
"modified" bit in the PTE.