[clang] [AMDGPU] Use the AMDGPUToolChain when targeting C/C++ directly (PR #99687)

[Joseph Huber via cfe-commits]

jhuber6 wrote:

> Does this change the behavior with amdgcn-- or amdgcn-mesa-mesa3d?

Those both use the `AMDGPUToolChain`. I suppose you could make the argument that `--target=amdgcn--` is the intended  target for "standalone amd" stuff.

https://github.com/llvm/llvm-project/pull/99687