arch-riscv,util: Add m5op.S for riscv to enable pseudo inst use

arch-riscv,util: Add m5op.S for riscv to enable pseudo inst use

This change adds assembly code for riscv pseudo instructions so
that they can be used with riscv benchmarks.

Change-Id: Ic979fd375e7750e92f52b900bf39e351f629fe2c
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/38515
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Jason Lowe-Power <power.jg@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>