arch-riscv: Move compressed ops out of ISA

This patch moves static portions of the compressed instruction
definitions out of the ISA generated code.

Change-Id: I61daae8b8c03a9e0f012790a132aa4d34a6ec296
Reviewed-on: https://gem5-review.googlesource.com/6026
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>