kexec: Introduce basic arm64 support

This works on older arm64 platforms, but may not work with arm64 devices
using GICv3, due to a quirk in the GICv3, where some registers are
write-once.

Most of the kexec reboot work on arm64 can be done entirely in C code,
by disabling the MMU, as the kernel is carved out of the vm_phys_segs
array, so cannot be overwritten.

Reviewed by: andrew
Sponsored by: Juniper Networks, Inc.
Differential Revision: https://reviews.freebsd.org/D51621