[S390] irq: set __ARCH_IRQ_EXIT_IRQS_DISABLED

Set __ARCH_IRQ_EXIT_IRQS_DISABLED in order to optimize irq_exit() a
bit, since we call __do_softirq() instead of do_softirq().
This saves several needless checks, pointless interrupt disabling
and an extra branch.
If do_softirq() gets called from process context we still switch to
the async stack.

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