Add pmap_enter(9) PMAP_ENTER_LARGEPAGE flag and implement it on amd64.

The flag requests entry of non-managed superpage mapping of size
pagesizes[psind] into the page table.

Pmap supports fake wiring of the largepage mappings. Only attributes
of the largepage mapping can be changed by calling pmap_enter(9) over
existing mapping, physical address of the page must be unchanged.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D24652