genirq: Implement generic irq_show_interrupts()

All archs implement show_interrupts() in more or less the same
way. That's tons of duplicated code with different bugs with no
value. Implement a generic version and deprecate show_interrupts()

Unfortunately we need some ifdeffery for !GENERIC_HARDIRQ archs.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>