arch-riscv: Enable support for riscv 32-bit in SE mode.

arch-riscv: Enable support for riscv 32-bit in SE mode.

This patch splits up the riscv SE mode support for 32 and 64-bit.
A future patch will add support for decoding rv32 instructions.

Change-Id: Ia79ae19f753caf94dc7e5830a6630efb94b419d7
Signed-off-by: Austin Harris <austinharris@utexas.edu>
Reviewed-on: https://gem5-review.googlesource.com/c/15355
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Alec Roelke <alec.roelke@gmail.com>
Maintainer: Alec Roelke <alec.roelke@gmail.com>