Rather than an "iointr" routine that decomposes a vector into an
IRQ, we maintain a vector table directly, hooking up each "iointr"
routine at the correct vector. This also allows us to hook device
interrupts up to specific vectors (c.f. Jensen).
We can shave even more cycles off, here, and I will, but it requires
some changes to the alpha_shared_intr stuff.
thorpej