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>
nbody_CPU_SOA.cpp: add SIMD vectorization support

The inner loop can be explicitly called out for vectorization.

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