Translate corefn.json in parallel

[felixSchl]

Update the mk/target.mk file to have a one-to-one mapping from corefn.json to a .c file, something like: %.c: %/corefn.json. That means Make can compile the corefn jsons into C files in parallel using its' -j flag.

[felixSchl]

Alternatively (or additionally), we can add support for translating corefn.jsons in parallel internally. It should be fairly easy to add this support and the current CLI already takes in a list of items to process (as opposed to just one), so it arguable falls in the scope of the utility.