amd64 pmap: rename _pmap_allocpte() to pmap_allocpte_alloc().

The function performs actual allocation of pte, as opposed to
pmap_allocpte() that uses existing free pte if pt page is already
there. This also moves function out of namespace similar to a language
reserved.

Reviewed by: markj
Tested by: pho
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D27956