[S390] topology: expose core identifier

Provide a topology_core_id define which makes sure that the contents of
/sys/devices/system/cpu/cpuX/topology/core_id
indeed do contain the core id and not always 0.

Signed-off-by: Heiko Carstens <heiko.carstens@de.ibm.com>
Signed-off-by: Martin Schwidefsky <schwidefsky@de.ibm.com>