Support attaching tx queues to cpus

This will attempt to use a different thread/core on the same L2
cache when possible, or use the same cpu as the rx thread when not.
If SMP isn't enabled, don't go looking for cores to use. This is mostly
useful when using shared TX/RX queues.

Reviewed by: sbruno
Sponsored by: Limelight Networks
Differential Revision: https://reviews.freebsd.org/D12446