riscv: Export the system call ABI for use in gem5 ops.

riscv: Export the system call ABI for use in gem5 ops.

This ABI is effectively used by both the gem5 ops and system calls, in
system calls because it only relies on registers, and in gem5 ops by
inheritance.

Even though these ABIs happen to be the same and were initially defined
to be the same, this change creates a root "reg" ABI which will act as a
root for both so that there isn't an implication that changes to one
should be changes to both.

Change-Id: I8726d8628503be2ad7616a71cc48b66f13e7d955
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39318
Reviewed-by: Jason Lowe-Power <power.jg@gmail.com>
Reviewed-by: Ayaz Akram <yazakram@ucdavis.edu>
Maintainer: Gabe Black <gabe.black@gmail.com>
Tested-by: kokoro <noreply+kokoro@google.com>