nbody: if OpenMP is enabled, find and add libiomp5 to libraries

libiomp5 is much better than libgomp, so we try to always link it explicitly
when it's available.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
nbody_CPU_{AOS,SOA}_tiled: handle non-power-of-two N correctly

I was noticing on a 24-thread Westmere box that the relative error was very
high on the tiled tests. This is because N % 1024 != 0. De-templatize the tiled
implementations, and use a dynamic tile size.

Signed-off-by: Steven Noonan <steven@uplinklabs.net>
chThread: use omp_get_num_threads for processorCount, if built with OpenMP

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