arm64: Start splitting out undef sys insn handling

We shouldn't need to decode the instruction when handling an unknown
SYS, MSR, or MRS instruction that raises an EXCP_MSR exception. The
exception syndrome (esr) contains all the information we need and is
safer to use than reading the instruction.

Add a new set of callbacks for these instructions that we can pass in
either the esr from hardware, or a generated version when we have to
fall back to instruction decoding.

Reviewed by: harry.moulton_arm.com
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D50208