Fix up the IRQ remapping code from the IOMMU controller driver in
order to actually ack the APIC upon incoming edge interrupts. A
previous change made ack_APIC_irq() a null placeholder, since the
original interrupt handlers calling it should not ack the APIC
anymore, once pipelining is in place.

We must revert this change in the acknowledge handler though, since
the pipeline core needs it to issue such ack.