[flang] Add .mod file dependencies

Make compilation of other predefined module files depend on
__fortran_builtins.mod. Currently only iso_c_binding.f90 and
iso_fortran_env.f90 depend on it but others could in the future.

Create the .f18.mod files by copying from the .mod files so that
we don't have to worry about dependencies for those.

Original-commit: flang-compiler/f18@8209ad3d32909573e9ef72327ba61932f8967587
Reviewed-on: https://github.com/flang-compiler/f18/pull/899