[CUDA][HIP] Add support for `--offload-arch=native` to CUDA and refactor

This patch adds basic support for `--offload-arch=native` to CUDA. This
is done using the `nvptx-arch` tool that was introduced previously. Some
of the logic for handling executing these tools was factored into a
common helper as well. This patch does not add support for OpenMP or the
"new" driver. That will be done later.

Reviewed By: yaxunl

Differential Revision: https://reviews.llvm.org/D141051