[S390] irqstats: split IPI interrupt accounting

We use both the external call and emergency call IPIs to signal remote
cpus. Therefore it makes sense to account them differently withing
/proc/irqstats so we actually know what happened.

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