nbody.cu: tickle OpenMP before starting measured workloads

Thread creation takes a while, there's no sense measuring it. Once we get
OpenMP invoked once, subsequent re-uses of the same thread pools are cheap. So
let's make OpenMP do something simple, like invoke a ton of tasks, and then get
the thread count.

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