[NVPTX] Introduce attribute to mark kernels without a language mode

We may want to be able to mark certain regions as kernels even without
being in an accepted CUDA or OpenCL language mode. This patch introduces
a new attribute limited to `nvptx` targets called `nvptx_kernel` which
will perform the same metadata action as the existing CUDA ones. This
closely mimics the behaviour of the `amdgpu_kernel` attribute. This
allows for making executable NVPTX device images without using an
existing offloading language model.

I was unsure how to do this, I could potentially re-use all the CUDA
attributes and just replace the `CUDA` language requirement with an
`NVPTX` architecture requirement. Also I don't know if I should add more
than just this attribute.

Reviewed By: tra

Differential Revision: https://reviews.llvm.org/D140226