Teach auto-generated interrupt handlers to clear interrupts again

[rswarbrick]

It would be nice if we could customise our (new) sequences that drive interrupts to drop the interrupts again only when they were acknowledged. (Ibex has level-, rather than edge-triggered interrupts)

At the moment, the ISRs that come out of riscv-dv write to a magic "signature address". We could either use that for this purpose or could use something else. But the latter case would mean a modification of the (moribund?) riscv-dv project.

estimate 4

[johngt]

@GregAC - checking if this is still relevant and if so, to whom we should assign this.

[GregAC]

Still relevant but something to do for V3 and it's already assigned to that milestone