[Flang-rt] Forward libomp-mod dependency for non-default runtime builds

[jhuber6]

Summary:
Currently this is required to inject the dependnecy so the object is
built when targeting OpenMP. This was only applied to the
runtime_register_default_target which only applies to
LLVM_ENABLE_RUNTIMES. The user could concievably do cross-compiling,
or just manually specify the host architecture and it would then error.

[Meinersbur]

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.

[jhuber6]

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.

FYI, this form doesn't always imply cross-compiling. You can do -DRUNTIMES_x86_64-unknown-linux-gnu_LLVM_ENABLE_RUNTIMES=flang-rt. However, it will add the handling for CMAKE_CXX_COMPILER_TARGET which might add the --target= flag, but I think that's handled in flang currently.

[Meinersbur]

Can it wait for after #137828 ? It will introduce different .mod locations per target, so they would not overwrite each other anymore.