[PowerPC64LE] Set up powerpc.powerpc64le architecture

This is the initial set up for PowerPC64LE.

The current plan is for this arch to remain experimental for FreeBSD 13.

This started as a weekend learning project for me and kinda snowballed from
there.

(More to follow momentarily.)

Reviewed by: imp (earlier version), emaste
Sponsored by: Tag1 Consulting, Inc.
Differential Revision: https://reviews.freebsd.org/D26399