arch,sim: Convert clone to GuestABI and define a cloneBackwardsFunc.

arch,sim: Convert clone to GuestABI and define a cloneBackwardsFunc.

cloneBackwardsFunc takes its arguments in the order specified for
ARM and RISCV. Because of the new GuestABI mechanism, it can be a
simple wrapper around the normal clone implementation without the need
for #ifdefs.

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

Change-Id: Iff1ffd6774b9162185a124585e9507a5bdbc46f4
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/23198
Reviewed-by: Bobby R. Bruce <bbruce@ucdavis.edu>
Maintainer: Gabe Black <gabeblack@google.com>
Tested-by: kokoro <noreply+kokoro@google.com>