[NVPTX] Add __CUDA_ARCH__ macro to standalone NVPTX compilations

We can now target the NVPTX architecture directly via
`--target=nvptx64-nvidia-cuda`. This currently does not define the
`__CUDA_ARCH__` macro with is used to allow code to target different
codes based on support. This patch simply adds this support.

Reviewed By: tra, jdoerfert

Differential Revision: https://reviews.llvm.org/D146975