[llvm] [offload] Fix finding amdgpu/nvptx-arch to generate tests (PR #135072)

[Joseph Huber via llvm-commits]

https://github.com/jhuber6 commented:

Weird, usually the binary dir thing refers to the CMake output directory, not literal `bin/`. This should be the path that's taken on a runtimes build, so I'm unsure how it's hitting that. Could be some LLVM magic, I'm not sure.

https://github.com/llvm/llvm-project/pull/135072