[OPENMP][NVPTX]Improve omp_get_max_threads() function.

Summary:
Function omp_get_max_threads() can always return 1 if current execution
mode is SPMD.

Reviewers: grokos, gtbercea, kkwli0

Subscribers: guansong, jdoerfert, caomhin, openmp-commits

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D61379

llvm-svn: 359792