[OPENMP][NVPTX]Add __kmpc_syncwarp(int32_t) function.

Summary:
Added function void __kmpc_syncwarp(int32_t) to expose it to the
compiler. It is required to fix the problem with the critical regions in
Cuda9.0+. We cannot use barrier in the critical region, but still need
to reconverge the threads in the warp after. This function allows to do
this.

Reviewers: grokos, jdoerfert

Subscribers: guansong, openmp-commits, kkwli0, caomhin

Tags: #openmp

Differential Revision: https://reviews.llvm.org/D66672

llvm-svn: 369933