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

[Joseph Huber via cfe-commits]

jhuber6 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)

OpenMP already defaults to `native` if you don't specify anything. Having the actual toolchain variant on commandline arguments makes me a little uncertain. Long term I'd prefer people do something like `--offload-targets=spirv64-amd-amdhsa` or something and that just defaults to the proper toolchain.

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