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

[Artem Belevich via cfe-commits]

Artem-B wrote:

> Right now if you specify target-cpu you get target-cpu attributes, which is what we don't want. 

I'm fine handling 'generic' in a special way under the hood and not specifying target-CPU.

My concern is about user-facing interface. Command line options must be overridable. 
For the CPU I would be able to specify the variant that matches the default.
For GPU I'll have no way to explicitly pick 'generic' as the target. I think this is important.




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