Remove all target-specific portions of Config.hs
- If GHC is to be multi-target, these cannot be baked in at compile time.
- Compile-time flags have a higher maintenance than run-time flags.
- The old way makes build system implementation (various bootstrapping details) with the thing being built. E.g. GHC doesn't need to care about which integer library *will* be used---this is purely a crutch so the build system doesn't need to pass flags later when using that library.
- Experience with cross compilation in Nixpkgs has shown things work nicer when compiler's can *optionally* delegate the bootstrapping the package manager. The package manager knows the entire end-goal build plan, and thus can make top-down decisions on bootstrapping. GHC can just worry about GHC, not even core library like base and ghc-prim!