[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

git-svn-id: https://llvm.org/svn/llvm-project/openmp/trunk@348491 91177308-0d34-0410-b5e6-96231b3b80d8