[clang] [NVPTX] Add `-march=general` option to mirror default configuration (PR #85222)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 requested changes to this pull request.

Thanks for looking at this. When the user compiles with `-march=xyz` it introduces a lot of subtarget specific metadata intro the output IR. The purpose of the original patch was to keep `-target-cpu` unset in cases where `-march=xyz` was not passed in. The expected semantics here is that `-march=sm_52 -march=generic` will override `-march=sm_52` and result in no `-target-cpu` being set just like if you didn't pass `-march` at all.

https://github.com/llvm/llvm-project/pull/85222