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

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> > It's just the AMDGCN target without any `+features`, right? The only issue I was aware of was assuming w64 when unspecified but you fixed that previously.
> 
> Almost, but it's problematic in several ways. The problems multiply once you start adding in manually specified target_features attributes

I don't do that anywhere so probably why I haven't encountered those issues (so far). Though I wonder how the proposed AMDGCN version of `__nvvm_reflect` Alex is working on will interact with that as well, since I think the SPIR-V target would share similar issues if we're emitting builtins that require certain target features?

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