genirq: Disable irqfixup/poll on PREEMPT_RT.

The support for misrouted IRQs is used on old / legacy systems and is
not feasible on PREEMPT_RT.

Polling for interrupts reduces the overall system performance.
Additionally the interrupt latency depends on the polling frequency and
delays are not desired for real time workloads.

Disable IRQ polling on PREEMPT_RT and let the user know that it is not
enabled. The compiler will optimize the real fixup/poll code out.

[ bigeasy: Update changelog and switch to IS_ENABLED() ]

Signed-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Sebastian Andrzej Siewior <bigeasy@linutronix.de>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Link: https://lore.kernel.org/r/20210917223841.c6j6jcaffojrnot3@linutronix.de