target/arm: Move setting of CPU halted state to generic code

This change is in a separate patch because it's not so obvious that it
won't cause a regression.

Suggested-by: Eduardo Habkost <ehabkost@redhat.com>
Reviewed-by: Philippe Mathieu-Daudé <philmd@redhat.com>
Reviewed-by: David Gibson <david@gibson.dropbear.id.au>
Reviewed-by: Greg Kurz <groug@kaod.org>
Signed-off-by: Thiago Jung Bauermann <bauerman@linux.ibm.com>
Message-Id: <20200826055535.951207-3-bauerman@linux.ibm.com>
Signed-off-by: David Gibson <david@gibson.dropbear.id.au>