[NVPTX] Set the atomic inling threshold when targeting NVPTX directly

Since Clang 16.0.0 users can target the `NVPTX` architecture directly
via `--target=nvptx64-nvidia-cuda`. However, this does not set the
atomic inlining size correctly. This leads to spurious warnings and
emission of runtime atomics that are never implemented. This patch
ensures that we set this to the appropriate pointer width. This will
always be 64 in the future as `nvptx64` will only be supported moving
forward.

Fixes: https://github.com/llvm/llvm-project/issues/61410

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D146750