arm64 pmap: Convert PC_IS_FREE into an inline function.

This permits inlining the comparisons even in the 16K page case.
Note that since PC_FREEN is -1, values can be compared efficiently
without having to fetch words of pc_freemask from memory via the
'cmn <reg>, #0x1' instruction.

Reviewed by: markj
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D36218