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

[Artem Belevich via cfe-commits]

Artem-B wrote:

Relying on something *not* being defined is probably not the best way to handle 'generic' target. For starters it makes it hard or impossible to recreate the same compilation state by undoing already-specified option. It also breaks established assumption that there *is* a default target CPU/GPU. If we do want to have a generic GPU target, then we should grow an explicit 'generic' GPU variant, IMO. It would be a functional opposite of 'native'.

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