[NVPTX] Add support for selecting CUDA vs OCL mode based on triple

IR for CUDA should use "nvptx[64]-nvidia-cuda", and IR for NV OpenCL should use "nvptx[64]-nvidia-nvcl"

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@184579 91177308-0d34-0410-b5e6-96231b3b80d8