[llvm] [AMDGPU] Fix unittest linking error with `LLVM_LINK_LLVM_DYLIB` (PR #91727)

[Jonas Hahnfeld via llvm-commits]

hahnjo wrote:

Hi @JanekvO, fully removing the unittest of course also works. I'm not really familiar with the AMDGPU target nor MC parts of LLVM, just happened to run into linking errors this morning.

https://github.com/llvm/llvm-project/pull/91727