[OPENMP][NVPTX]Fix __kmpc_flush to flush the memory per system, not per block.

Summary:
According to the standard, after memory flushing the changes in the
memory must be visible to all the threads in all teams. Patch fixes
this.

Reviewers: gtbercea, kkwli0

Subscribers: guansong, jfb, caomhin, openmp-commits

Differential Revision: https://reviews.llvm.org/D55370

llvm-svn: 348491