[clang] [Offload] Treat an empty packager archicture as 'generic' (PR #126655)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> What happens with `-march` not specified? I only know about gcc falling back on `-march=x86-64`, but depends on the target triple.

Same thing that happens when you do `--target=x86_64-unknown-linux-gnu` without any `-march` option. For the GPU it defaults to like gfx700 or something, but that's not really our fault since the user never specified one.

https://github.com/llvm/llvm-project/pull/126655