bpo-47146: Stop Depending On regen-deepfreeze For regen-global-objects (gh-32218)

This effectively reverts the Makefile change in gh-31637. I've added some notes so it is more clear what is going on.

We also update the "Check if generated files are up to date" job to run "make regen-deepfreeze" to ensure "make regen-global-objects" catches deepfreeze.c.

https://bugs.python.org/issue47146