[OPENMP][NVPTX]Use __kmpc_barrier_simple_spmd(nullptr, 0) instead of

[OPENMP][NVPTX]Use __kmpc_barrier_simple_spmd(nullptr, 0) instead of
nvvm_barrier0.

Use runtime functions instead of the direct call to the nvvm intrinsics.
It allows to prevent some dangerous LLVM optimizations, that breaks the
code for the NVPTX target.

llvm-svn: 350328