Provided Toolchain Support

Added support for provided toolchains by invoking fatso_system_with_callbacks with exact content of `config.toolchain`. This bypasses the attempt to guess the toolchain, because `config.toolchain` is set.

Question before this gets merged: in `fatso_guess_toolchain`: `out_toolchain->name` is currently set to `"[config provided]"`, the other options are `fatso_package->toolchain`, or another string. Is this the best choice?