[libomptarget-nvptx] Fix number of threads in parallel

If there is no num_threads() clause we must consider the
nthreads-var ICV. Its value is set by omp_set_num_threads()
and can be queried using omp_get_max_num_threads().
The rewritten code now closely resembles the algorithm given
in the OpenMP standard.

Differential Revision: https://reviews.llvm.org/D51783

llvm-svn: 343380