Make ghc-toolchain vs configure differences errors instead of warnings
I believe ghc-toolchain has been introduced for over 2 releases now, and that we should proceed to the next stage: making it the default and deleting toolchain configuration logic from configure.
To this end, we should try to make the current configure warning which diffs the outputs of configure and ghc-toolchain into errors. Given the lack of recent reports about toolchains differences and the fact that CI currently checks for these differences as errors, they should not be terribly common and push remaining problems to be reported...
Although this may be too heavy of a hammer. It would be good to discuss what should be the path forward for a ghc-toolchain-only-future with high assurance.