This generalizes {ARM,M68k,Alpha}CPUListState to avoid declaring it for
each target. Place it in cpu-common.h to avoid circular dependencies.

Signed-off-by: Andreas Färber <afaerber@suse.de>
Reviewed-by: Igor Mammedov <imammedo@redhat.com>
Reviewed-by: Eduardo Habkost <ehabkost@redhat.com>