[llvm] [Offload] Fix PowerPC builds that pass -mcpu (PR #138327)

[Nikita Popov via llvm-commits]

nikic wrote:

Another variant of this issue:
```
clang++: error: unsupported option '-mtls-dialect=' for target 'amdgcn-amd-amdhsa'
```

https://github.com/llvm/llvm-project/pull/138327