arm64: Reduce where we decode msr/mrs instructions

[andrew]

Summary

We only use the mrs_Op* and mrs_CR* functions and MRS_* macros when
handling userspace executing a msr/msr instruction.

Move the macros to where they are used and expand the functions to
just use the macros directly.

While here update MRS_Op0_MASK to include bit 20 as this will cause
the correct op0 value to be calculated for all instructions we decode.

Sponsored by: Arm Ltd

[andrew]

Rebase on D50906 to move all MRS_ macros to undefined.c