Despite our efforts to re-implement the toolchain bits of the configuration script faithfully in ghc-toolchain, there are likely some bugs still lurking on some platforms. We should test CI on all machines with the configure flag `--enable-ghc-toolchain` and fix the lurking bugs. Potentially, we could also add a CI job in which we use `--enable-ghc-toolchain` by default (but on which platform?) Additionally, (or alternativelly), we should check the output of the validation between configure and ghc-toolchain. NB: During development in !9263 we came across multiple tasks and bugs regarding the configuration script. In preparation of merging it, we're opening tickets for the leftover tasks both regarding the new `ghc-toolchain` and the toolchain logic in `configure`. This is one of those tickets.