Re-generate compile_commands.json incrementally

[amezin]

Generate a .json file for every .cmd.* file, then join them. So the Python script will only handle one .cmd.* at a time, and a Makefile will run the script for all files.

1. .json files for unchanged .cmd files won't be regenerated needlessly.
2. make's globbing/wildcard is probably faster than Python's one.
3. this will be closer to something that can be integrated into Kbuild
4. Probably it'll be possible to trick kernel's Makefile to include/recursively call this Makefile (and thus automatically pass correct O=..., for example)