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

[Alex Voicu via cfe-commits]

AlexVlx wrote:

I think that in general we also need to decide on what happens when you pick an amdgcn— triple. IMHO for that case we should probably error out if no mcpu is provided, since there’s no reasonable default, except for “all”, but that would be incredibly disruptive.

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