nbody: remove exports for non-exported symbol sseWorkerThread()

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: make OpenMP version simpler

Might as well use the nbody_CPU_SSE code as a base for this, since OpenMP is
taking care of the threading bits.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>