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

This PR adds `-march=generic` support for the NVPTX backend. This
fulfills a TODO introduced in #79873.

With this PR, users can explicitly request the "default" CUDA
architecture, which makes sure that no specific architecture is
specified.

This PR does not address any compatibility issues between different CUDA
versions.

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Co-authored-by: Joseph Huber <huberjn@outlook.com>