[flang] [llvm] [flang-rt] Add experimental support for GPU build (PR #131826)

[Michael Kruse via llvm-commits]

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

LGTM.

The `gpu_sources` list will probably evolve over time, fixing more and more. It might actually a good idea to have a separate list until it stabilized.

https://github.com/llvm/llvm-project/pull/131826