[clang] [HIP] change default offload archs (PR #139281)

[Artem Belevich via cfe-commits]

Artem-B wrote:

@jhuber6 do you think can we use `native` instead? I think it would be a somewhat better option here.
If we have to choose a GPU variant by default, we may as well choose the actual GPU, rather than a conditional choice between generic SPIR-V or an old GPU, which has the disadvantage of not being the right choice for anyone, and depending on another moving part (amd-llvm-spirv)

https://github.com/llvm/llvm-project/pull/139281