nbody: make ICC builds less chatty

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: silence warning about implicit conversion from size_t to int

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody_CPU_AOS: drop 'shared' specifier on OpenMP stanza

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: change tile size for CPU_*_tiled to be greatest common factor

We do multiples of 1024 bodies, so it makes sense to set our tiles to that
size. In some quick testing, this reduced the time cost for the _tiled
variants.

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