arm64: Fix the MRS_EXACT_IF_DIFFERENT check

When comparing two ID registers to create a common userspace view we
have an option where we set an exact value only if the two are
different. This is only used in one field in the cache type register
that is accessible by userspace without being trapped by the kernel.

In preparation for trapping access to this register to handle it in
the kernel in some situations fix the 'if different' check.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D47811