There are edge triggered interrupt controllers around where the
"enable" control is before the edge detection.  This means that
we must keep the interrupts enabled, even when disable_irq() is
in effect, and mark such interrupts for later processing.

To make matters worse, some of these interrupt controllers do not
provide any method to re-trigger the interrupt.  We are unable
to call the handler direct from enable_irq() since we may be
in an IRQ-protected context.

Therefore, we add such interrupts to a list of pending interrupts,
which we process at the next hardware interrupt.