nbody_CPU_AOS_tiled: make functions static

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: make pos/mass arguments to ComputeGravitation constant

This prevents us from making errors in our code to an extent, and also gives
the compiler a bit more room to optimize.

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