[OPENMP][NVPTX] Add support for lastprivates/reductions handling in SPMD...

[OPENMP][NVPTX] Add support for lastprivates/reductions handling in SPMD constructs with lightweight runtime.

Summary:
We need the support for per-team shared variables to support codegen for
lastprivates/reductions. Patch adds this support by using shared memory
if the total size of the reductions/lastprivates is <= 128 bytes,
then  pre-allocated buffer in global memory if size is <= 4K bytes,or
uses malloc/free, otherwise.

Reviewers: gtbercea, kkwli0, grokos

Subscribers: guansong, openmp-commits

Differential Revision: https://reviews.llvm.org/D51875

llvm-svn: 342737