arch-riscv: Move unknown out of ISA description

arch-riscv: Move unknown out of ISA description

This patch removes the Unknown instruction type out of the ISA generated
code and puts it into arch/riscv/insts.  Since there isn't any dynamic
behavior to it, all that's left behind is a template for creating a new
Unknown instruction.

Change-Id: If7c3258a24ecadd3e00ab74586e1740e14f028db
Reviewed-on: https://gem5-review.googlesource.com/6023
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>