Define _NPCM and the last PC_FREEn constant in terms of _NPCPV.

This applies one of the changes from
5567d6b4419b02a2099527228b1a51cc55a5b47d to other architectures
besides arm64.

Reviewed by: kib
Sponsored by: DARPA
Differential Revision: https://reviews.freebsd.org/D36263