bpo-40528: Improve AST generation script to do builds simultaneously (GH-19968)

bpo-40528: Improve AST generation script to do builds simultaneously (GH-19968)

- Switch from getopt to argparse.
- Removed the limitation of not being able to produce both C and H simultaneously.

This will make it run faster since it parses the asdl definition once and uses the generated tree to generate both the header and the C source.