amd64 pmap: simplify vtopte() and vtopde()

Pre-calculate masks and page table/page directory bases, for LA48 and
LA57 cases, trading a conditional for the memory accesses.

This shaves 672 bytes of .text, by the cost of 32 .data bytes. Note
that eliminated code contained one conditional, and several costly
movabs instructions, which are traded by two memory fetches per
vtop{t,d}e() inlines.

Reviewed by: markj
Discussed with: alc
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33776