[llvm] [CodeGen] Gate rematerializer unit tests on AMDGPU target being enabled (PR #202966)

[Paul Osmialowski via llvm-commits]

pawosm-arm wrote:

I'm still seeing this in our CI:
```
17:54:25  # | /workspace/src/flang/test/Driver/target-cpu-features.f90:79:17: error: CHECK-AMDGPU: expected string not found in input
17:54:25  # | ! CHECK-AMDGPU: "-fc1" "-triple" "amdgcn-amd-amdhsa"
```

https://github.com/llvm/llvm-project/pull/202966