arch/*: Disable softirq stacks on PREEMPT_RT.

PREEMPT_RT preempts softirqs and the current implementation avoids
do_softirq_own_stack() and only uses __do_softirq().

Disable the unused softirqs stacks on PREEMPT_RT to save some memory and
ensure that do_softirq_own_stack() is not used bwcause it is not expected.

Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Link: https://lkml.kernel.org/r/CAK8P3a1QmeAscV-Ory-Dae4RoLvDSPEjEgFGQHR9U8jUervGuA@mail.gmail.com