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: Add bhyverun and vmexit handlers for arm64

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