[OpenMP][libomptarget] Add function for checking SPMD mode

Summary: Add function to the NVPTX libomptarget library that will return true if the current target region is being executed in SPMD mode.

Reviewers: ABataev, grokos, carlo.bertolli, caomhin

Reviewed By: grokos

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D46840

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@332360 91177308-0d34-0410-b5e6-96231b3b80d8