drop _OPENMP dependency for SIMD

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>