[OPENMP][NVPTX]Fixed initialization of the data-sharing interface.

Summary:
Avoid using of the atomic loop to wait for the completion of the
data-sharing interface initialization, use __shfl_sync instead for the
communication within the warp to signal other threads in the warp about
completion of the initialization.

Reviewers: gtbercea, kkwli0, grokos

Subscribers: guansong, jfb, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D56100

llvm-svn: 350129