nbody_CPU_AOS: add condition to avoid calculating against self

The 'i == j' condition was impossible in nbody_CPU_SOA, but not here -- it's
definitely possible for us to compare an element against itself, which makes no
sense to do.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody.cu: respect 'nosimd' argument even without crosscheck

It affects other things (such as which algorithms to cycle through when hitting
the spacebar). There's no reason to disrespect it when crosscheck is off.

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