SPDX and reuse lint

[XVilka]

To verify the license and copyright of every file and that everything in order in every PR:

• https://reuse.software/
• https://github.com/fsfe/reuse-tool/
• https://github.com/marketplace/actions/reuse-compliance-check

[Rot127]

Generated files and LLVM files should not be licensed via a comment in the file, but via the dep5. It is way easier and saves us again some code generation trouble.
Also, currently the copyright header is plainly wrong if anyone else then me or @aquynh submits code. Which is luckily provable thanks to git/Github.