Add tunable to allow interrupts on hyperthreaded cores

[gallatin]

Summary

This benefits workloads which are primarily interrupt driven by increasing the logical cores available for interrupt handling.

Note that there is a review open which will make the ULE scheduler schedule an ithread immediately on the local core. (https://reviews.freebsd.org/D17885). Running a Netflix workload with this patch applied leads to very uneven CPU use, as a substantial fraction of our workload is interrupt driven from the NIC ithread, and the logical cores are underutilized.