emulators/linux-power9-functional-sim: New port

This is a full system simulator for IBM's POWER9 and is sufficent to run
FreeBSD 12.0 or POWER9BSD work in progress. Ask in #powerpc64 on efnet for
usage help for now.

Approved by: timur (mentor)
Differential Revision: https://reviews.freebsd.org/D17545