build: reduce chance of unneeded rebuild

Run `node_js2c` and `mkssldef` as actions and not as targets makes sure
they are run only once, just before processing the rest of `node_lib`.
This helps `make` based dependency change detection be more accurate.

Add comments with tagrget names for readability.

Use `process_outputs_as_sources` for automatic inclution of outputs.

PR-URL: https://github.com/nodejs/node/pull/23156


Reviewed-By: Richard Lau <riclau@uk.ibm.com>
Reviewed-By: Joyee Cheung <joyeec9h3@gmail.com>