Hadrian doesn't (easily) build on platforms that don't support the threaded RTS

Today a s390 user came to #ghc saying that Hadrian wasn't building for him. The problem is that Hadrian's Cabal file includes -threaded -with-rtsopts -gq. However, s390 doesn't support the threaded RTS.

Removing -qg from the Cabal file fixed the issue however we really shouldn't require the user to hack the Cabal file just to build Hadrian. Perhaps we should add an on-by-default threaded Cabal flag which the user can flip off by command-line argument/environment variable/automagic as needed?