arm: Export the mostly generic syscall ABI.

arm: Export the mostly generic syscall ABI.

This ABI is also applicable for gem5 ops. Rather than have the gem5 ops
use the syscall ABI, this change exports the syscall ABI and renames it
the "reg" ABI, or in other words an ABI which only uses registers. The
SE workload class then just creates a local name for the "reg" ABI so it
can continue to use it for system calls.

Change-Id: Ifaa38a94d6f0d49b8a2e515e02ce94472a499a00
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/39315
Reviewed-by: Giacomo Travaglini <giacomo.travaglini@arm.com>
Maintainer: Giacomo Travaglini <giacomo.travaglini@arm.com>
Tested-by: kokoro <noreply+kokoro@google.com>