[OPENMP][NVPTX]Basic support for reductions across the teams.

Summary:
Added functions __kmpc_nvptx_teams_reduce_nowait_simple and
__kmpc_nvptx_teams_end_reduce_nowait_simple to implement basic support
for reductions across the teams.

Reviewers: gtbercea, kkwli0

Subscribers: guansong, jfb, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D54967

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@347710 91177308-0d34-0410-b5e6-96231b3b80d8