bhyve: Implement a PL031 RTC on arm64

Unlike amd64's, this RTC is implemented entirely in userspace. This is
the same RTC as is provided by QEMU's virt machine.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Support legacy PCI interrupts on arm64

This allows us to remove various #ifdef hacks and enable building more
PCI devices.

Note that a hole is left in the interrupt mapping for the RTC rather
than having the two core devices straddle the PCIe interrupts. QEMU's
virt machine also takes this approach.

Reviewed by: jhb
MFC after: 2 weeks
Obtained from: CheriBSD

bhyve: Push option parsing down into bhyverun_machdep.c

After a couple of attempts I think this is the cleanest approach despite
the expense of some code duplication. Quite a few of the single-letter
bhyve options are x86-specific.

I think that going forward we should strongly discourage the addition of
new options and instead configure guests using the more general
configuration file syntax.

Reviewed by: corvink, jhb
MFC after: 2 weeks
Sponsored by: Innovate UK
Differential Revision: https://reviews.freebsd.org/D41753

bhyve: Add bhyverun and vmexit handlers for arm64

Reviewed by: corvink, jhb
MFC after: 2 weeks
Differential Revision: https://reviews.freebsd.org/D41006