arm64: Use ISS to search for a special register

Previously we would use part of the msr/mrs instruction to find what
register was being asked for. This was the only use for these values.

We now have ESR_EL1 ISS field values for the same register so can use
this new value to find what register is asked for. This lets us remove
the old reg field from struct mrs_user_reg.

Add macros to keep the old KPI.

Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50905