arch-riscv: Move static_inst into a directory

arch-riscv: Move static_inst into a directory

This patch creates an "insts" directory in src/arch/riscv to store
static portions of instruction definitions that aren't part of the code
generated by the ISA description.  It serves as a starting point for
future patches to simplify the ISA description.

Change-Id: I6700522143f6fa6c9b18a30e1fbdc8f80cdc7afa
Reviewed-on: https://gem5-review.googlesource.com/6021
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Alec Roelke <ar4jc@virginia.edu>