arch-riscv: Simplify amd merge RV32/RV64 the RVM instructions

The change move the details implementation to utility.hh and merge
the RV32 and RV64 versions into one.

Change-Id: I438bfb0fc511f0f27e83f247d386c58493db65b4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/70597
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Yu-hsin Wang <yuhsingw@google.com>
Maintainer: Bobby Bruce <bbruce@ucdavis.edu>
2 files changed