Interrupt counters on `struct intr_event`

[ehem]

Every architecture has counters just outside struct intr_event. As such it seems appropriate to try to move them onto struct intr_event to reduce differences between architectures.

There is also an issue of the interface between architecture and core. The core has been implementing hw.intrcnt and hw.intrnames, yet forcing architectures to handle the tables behind these. This results in rather troublesome mixing.