alpha: Use generic show_interrupts()

The only subtle difference is that alpha uses ACTUAL_NR_IRQS and
prints the IRQF_DISABLED flag.

Change the generic implementation to deal with ACTUAL_NR_IRQS if
defined.

The IRQF_DISABLED printing is pointless, as we nowadays run all
interrupts with irqs disabled.

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