here is a patch set to add mold linker.
I've temporarily put the package definition into gnu/packages/raspberry-pi.scm as I'm unsure if it should go to gnu/packages/commencement.scm or somewhere else.
It would be be interesting to test it as a new default linker on such a large scale project.
Also there is currently one test failing so I've disabled the tests for now:
Testing mold-wrapper ... execl: No such file or directory