arm64/vmm: Add breakpoint and single-stepping support

This will be used to implement parts of bhyve's gdb stub.

Three VM capabilities are added, similar to amd64 without monitor mode.
Two cause breakpoint and single-step exceptions to be raised to EL2 and
then down to bhyve. One lets the gdb stub mask hardware interrupts
while single-stepping, since otherwise the guest will handle a timer
interrupt before executing the target instruction and thus fail
to make progress.

Reviewed by: bnovkov, andrew
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D44739

Import the kernel parts of bhyve/arm64

To support virtual machines on arm64 add the vmm code. This is based on
earlier work by Mihai Carabas and Alexandru Elisei at University
Politehnica of Bucharest, with further work by myself and Mark Johnston.

All AArch64 CPUs should work, however only the GICv3 interrupt
controller is supported. There is initial support to allow the GICv2
to be supported in the future. Only pure Armv8.0 virtualisation is
supported, the Virtualization Host Extensions are not currently used.

With a separate userspace patch and U-Boot port FreeBSD guests are able
to boot to multiuser mode, and the hypervisor can be tested with the
kvm unit tests. Linux partially boots, but hangs before entering
userspace. Other operating systems are untested.

Sponsored by: Arm Ltd
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation
Sponsored by: University Politehnica of Bucharest
Differential Revision: https://reviews.freebsd.org/D37428