[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

llvm-svn: 332360