[clang] [AMDGPU][OpenMP] Do not attach -fcuda-is-device flag for AMDGPU OpenMP (PR #96909)

[Joseph Huber via cfe-commits]

https://github.com/jhuber6 approved this pull request.

We don't even pass this in the NVPTX offloading case, so there's no reason to do it for AMDGPU.

https://github.com/llvm/llvm-project/pull/96909