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