Move the arm64 DMAP creation to C

To simplify the creation of the direct map (DMAP) region on arm64 move
it from the pre-C code into pmap. This simplifies the DMAP creation
as we can use the notmal index macros, and should reduce the number
of pages needed to hold the level 1 tables to just those needed.

Reviewed by: alc, dch
Tested by: dch, kevans
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D34568