[OPENMP][NVPTX] Lightweight runtime support for SPMD mode.

Summary:
Implemented simple and lightweight runtime support for SPMD mode-based
constructs. It adds support for L2 sequential parallelism wihtout full
runtime support. Also, patch fixes some use cases for
uninitialized|lightweight runtime.

Reviewers: grokos, kkwli0, Hahnfeld, gtbercea

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D51222

llvm-svn: 340944