[clang] [NVPTX] Allow compiling LLVM-IR without `-march` set (PR #79873)

[Artem Belevich via cfe-commits]

Artem-B wrote:

Considering that it's for the stand-alone compilation only, I'm not going to block this patch.
That said, please add a `TODO` somewhere to address an issue w/ explicitly targeting generic variant.
 

https://github.com/llvm/llvm-project/pull/79873