Enable build with LLVM toolchain provider via grailbio/bazel-toolchain

Historically m4 had difficulty building with this enabled, now it looks like the immediate issue is with the `dependency_support:embedded_python_interpreter` target e.g. running the following wildcard:

```console
$ bazel test -c opt --incompatible_enable_cc_toolchain_resolution ...
```

If we want to we could start trying to build up CI for targets that are known to work, but honestly running vs the standard CC toolchain on select OS-es has been working ok for development to date, so marking as "long term enhancement", just know that we're not winning any "hermiticity of the year" awards for our builds.