s390/runtime_instrumentation: re-add signum system call parameter

Add the signum system call parameter for documentation purposes only,
and without checking if the passed value is a valid real-time signal.

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