Add exported_linker_flags to cgo_library

[linzhp]

Now cgo_library can take C files in its srcs, we should add exported_linker_flags too, so the linker flags can be passed down to the final linking stage. Alternatives, we may pass all linker flags of cgo_library to all rules that transitively depend on it.

@mkaczanowski What do you think?

[mkaczanowski]

I don't mind having exported_linker_flags in cgo_library (that is already a customized cxx_library flavor). Feel free to come up with PR