[OPENMP][NVPTX]Improved omp_get_thread_limit() function.

Summary:
Function omp_get_thread_limit() in SPMD mode can return the maximum
available number of threads as a result.

Reviewers: grokos, gtbercea, kkwli0

Subscribers: guansong, jdoerfert, openmp-commits, caomhin

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D61378

llvm-svn: 359790