ghc-toolchain: Uncover bugs by configuring with ghc-toolchain by default in CI

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 (merged) 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.