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

[Matt Arsenault via cfe-commits]

arsenm 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 

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