powerpc64: Add initial support for HTM (kABI)

This patch adds the very initial support for HTM that might come at FreeBSD
version 12.1. This basic support defines a new kABI, so, we do not need to change
it later during 12.1 time frame, when the full implementation will come.

Reviewed by: jhibbits
Approved by: re(marius), jhibbits (mentor)
Differential Revision: https://reviews.freebsd.org/D16889