s390/smp: implement arch_irq_work_raise()

The immediate need to have this is to have bpf_send_signal() send the
signal ASAP instead of during the next hrtimer interrupt. However, it
should also improve irq_work_queue() latencies in general, as well as
get s390 out of the lame architectures list [1].

[1] https://git.kernel.org/pub/scm/linux/kernel/git/torvalds/linux.git/tree/kernel/irq_work.c?h=v5.11#n45



Signed-off-by: Ilya Leoshkevich <iii@linux.ibm.com>
Reviewed-by: Heiko Carstens <hca@linux.ibm.com>
Signed-off-by: Vasily Gorbik <gor@linux.ibm.com>