eliminate _threaded and _openmp variants, make stock variants OpenMP

If we build without OpenMP or use OMP_NUM_THREADS=1, you can see what
performance is like with a single thread.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: remove outdated build instructions from header

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: introduce CPU_SOA_tiled implementation

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody_CPU_AOS_tiled: add SIMD vectorization support to DoDiagonalTile

DoNonDiagonalTile is a more complicated story...

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody_CPU_SOA: make not suck

I don't know why this was doing it the way it was, but the atomic updates were
really slowing everything down.

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