LLM policy

LLM policy as proposed on sdcc-devel:

1. LLM use for creating testcases is fine.
2. LLM use for creating code other than testcases: the code should be reviewed by an SDCC developer other than the human who used the LLM before it gets committed to trunk.

Practically, I think 2 implies that such contributions should be small, to make them easy to review.