arm64: Use a bit to hold userspace ID reg exports

When exporting ID registers to userspace we should either adjust them
the same way for FreeBSD and Linux binaries, or export the Linux
field values to a safe value, e.g. when a needed syscall is missing.

To allow for this, and to clean up ID register handling in the kernel
move to using a bit per userspace ABI.

Reviewed by: imp
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47121