Add hypervisor trap handling, using HSRR0/HSRR1

Summary:
Some hypervisor exceptions on POWER architecture only save state to HSRR0/HSRR1.
Until we have bhyve on POWER, use a lightweight exception frontend which copies
HSRR0/HSRR1 into SRR0/SRR1, and run the normal trap handler.

The first user of this is the Hypervisor Virtualization Interrupt, which targets
the XIVE interrupt controller on POWER9.

Reviewed By: nwhitehorn
Differential Revision: https://reviews.freebsd.org/D15487