nbody: parallelize CPU_SOA with OpenMP

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody: make CPU_AOS vectorizable in ICC

It wasn't correctly vectorizing with #pragma simd before, because the 'acc'
variable was an array. Breaking it out into x/y/z seems to make it vectorize
correctly and vastly improve throughput.

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