Temporarily widen count for interrupt rate calculations on 32-bit archs

If the interrupt count is very high (greater than ~42M), notably on one-shot
execution on long running systems, the intermediate multiplication step in the
rate calculation will overflow the width of a 32-bit architecture long (32
bits), causing the rest of the calculation to calculate with a truncated value,
and report very low rates (sometimes 0).

MFC after: 2 weeks