sim: Add a transitional syscall ABI which defers to Process.

sim: Add a transitional syscall ABI which defers to Process.

This change adds a transitional ABI which just falls back to the
existing Process syscall arg getters. It should be phased out once each
ISA implements its own actual ABI.

Jira Issue: https://gem5.atlassian.net/browse/GEM5-187

Change-Id: Ic40bd924989f91de70bbce59fda888b79bbbfca4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23190
Reviewed-by: Gabe Black <gabeblack@google.com>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>