Powerpc64: Add the facility unavailable trap subsystem

Summary:
This code adds the basic infrastructure for the facility subsystem. A facility
trap is raised when an unavailable instruction is executed. One example is
executing a Hardware Transactional Memory instruction while the MSR[TM] is
disabled. In the past, there was a specific interrupt for it (FP, VEC), but the
new instructions seem to be multiplexed on this facility interrupt.

The root cause of the trap is provided on Facility Status and Control Register
(FSCR) register.

Submitted by: Breno Leitao
Reviewed by: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D14566