STMicroelectronics released their header definitions under more permissive licensing terms. Now this is possible.
If you are OK with their license, the migration to official header definitions is recommended. Otherwise, check the master branch of this repository. I do not expect the branch to be updated but it will still be around and usable.
If you prefer least intrusive patches to the official headers for SDCC support, I recommend my own flavor of the header.
Head to the new repository at
STM8S header