bhyve/arm64: Stop assuming the CPU index is MPIDR

We need the MPIDR value in a few places in userspace. Rather than
calculate it ask the kernel to give it to us. This allows us to change
how it is calculated without having to change userspace.

Reviewed by: markj
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D51763