Store mpidr as a 64-bit value on arm64

The mpidr register is 64 bit on arm64 and 32 bit on arm. Fix this by
extending the arm64 definition to include the top 32 bits.

To preserve KBI when MFCing split the value into two 32 bit values.
This will be cleaned up later only on main.

Reviewed by: bz
Sponsored by: The FreeBSD Foundation
Differential Revision: https://reviews.freebsd.org/D36346