The arm:compilers/arm/llvm-embedded (21.1.1) tarball on Darwin has a tarball file structure that doesn't match the `artifact.json`. The `./bin` is under an extra `ATfE-21.1.1-Darwin-universal` directory in the tarball. The Windows and Linux archives don't have this additional directory and the cmsis-toolbox tools work on those platforms without modification.
Manually updating the installation on Darwin to reflect the proper file layout--by moving everything up a level in the artifact directory--allows `csolution list toolchains` and `cbuild` to find the toolchain.
Thanks for reporting this, the Engineering team are now aware and this will be fixed in the next release