In the "Unix Makefiles" generator, the `ADDTIONAL_CLEAN_FILES` directory
property was evaluated on a per target basis. This had two drawbacks:
- per directory clean files were repeated in every target clean script
- per directory clean files weren't removed in directories without targets
  (issue #8164)

This patch moves the `ADDTIONAL_CLEAN_FILES` directory property processing
from the target to the directory level clean target.

Fixes: #8164 "ADDITIONAL_CLEAN_FILES directory property not respected if no
              target present in directory"