riscv: Convert RISCV to use local reg index storage.

riscv: Convert RISCV to use local reg index storage.

This was mostly straightforward, except that the micro and macro op
classes need to be seperated for AMO classes so that the reg_idx_arr_decl
will have the right sizes.

Change-Id: Ibc0a9df0cb79924342eaceb0f09606913442f841
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/36881
Reviewed-by: Gabe Black <gabe.black@gmail.com>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>