[flang] Small .mod file fixes

- Only emit initializations for parameters and derived type components.
- Use `=>` for pointer initializations.
- Don't emit intrinsic symbols.

Original-commit: flang-compiler/f18@b91748c053a11349f85645c65b870fab222ea135
Reviewed-on: https://github.com/flang-compiler/f18/pull/650
Tree-same-pre-rewrite: false