Fix use distro toolchian

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This is a small regression in making GHC multi-target, in that this should have gone in the settings file and not ghcplatform.h like the other Settings* AC_SUBST vars so it is not baked into the GHC binary.

@nrnrnr Fixing this might be a great first task, indeed it is the same sort of task I started with getting to know the current build systems.

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mentioned in merge request !6987 (merged)

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