Implement the FreeBSD specific pieces for thread affinity for OpenMP.

Upstream gcc 4.8 doesn't have support for this - it'll create threads,
but it won't do any of the thread affinity stuff for FreeBSD.

This allows for OMP_PROC_BIND=true to bind threads to their initial
CPUs, leading to some pretty drastic improvements in performance
for certain NUMA workloads.

Approved by: gerald