[S390] lowcore: Change type of lowcores softirq_pending to __u32.

As noted by akpm:

> kernel/time/tick-sched.c: In function 'tick_nohz_stop_sched_tick':
> kernel/time/tick-sched.c:229: warning: format '%02x' expects type 'unsigned int', but argument 2 has type '__u64'
>
> I don't think the architecture's local_softirq_pending() should return u64.
> This is the sort of thing which should be consistent across architectures.

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