Put the arm64 vttbr_el2 register into a state

Zero the vttbr_el2 register on each CPU so we can tell if we are
running the host or guest kernel from a hypervisor.

Obtained from: https://github.com/FreeBSD-UPB/freebsd-src (earlier version)
Sponsored by: Innovate UK
Sponsored by: The FreeBSD Foundation