[llvm] [Flang-rt] Forward `libomp-mod` dependency for non-default runtime builds (PR #137035)

[Michael Kruse via llvm-commits]

https://github.com/Meinersbur commented:

Cross-compiling using Flang currently is not supported. Another problem is that each `LLVM_RUNTIME_TARGETS` will write their own `omp_mod.lib` to the same location, so it was intentional that only the default target would do it. The flang tests assume the default target anyway.

https://github.com/llvm/llvm-project/pull/137035