igc: Add AIM

[kbowling]

Summary

We originally left this out because iflib modulates interrupts and
accomplishes some level of batching versus the custom queues in the
older driver. Upon more detailed study of the Linux driver which has a
newer implementation, it finally became clear to me this is actually a
holdoff timer and not an interrupt limit as it is conventionally
(statically) programmed and displayed as an interrupt rate. The data
sheets also make this somewhat clear.

Thus, AIM accomplishes two beneficial things for a wide variety of
workloads[1]:

1. At low throughput/packet rates, it will significantly lower latency
(by counter-intuitively "increasing" the interrupt rate.. better
thought of as decreasing the holdoff timer because you will modulate
down before coming anywhere near these interrupt rates).
2. At bulk data rates, it is tuned to achieve a lower interrupt rate
(by increasing the holdoff timer) than the current static 8000/s. This
decreases processing overhead and yields more headroom for other work
such as packet filters or userland.

For a single NIC this might be worth a few sys% on common CPUs, but may
be meaningful when multiplied such as if_lagg, if_bridge and forwarding
setups.

The AIM algorithm was re-introduced from the older igb or out of tree
driver, and then modernized with permission to use Intel code from other
drivers.

[1]: http://iommu.com/datasheets/ethernet/controllers-nics/intel/e1000/gbe-controllers-interrupt-moderation-appl-note.pdf

Test Plan

Tested on igc(4)