Allow the kernel to emulate the physical counter on arm64

When running under a VM we don't have access to the physical counter.
Add support to emulate this instruction by handling the trap in the
kernel. As it is slow only enable when the hw.emulate_phys_counter
tunable is set on boot.

Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D35613