Remove all target-specific portions of Config.hs

1. If GHC is to be multi-target, these cannot be baked in at compile
   time.

2. Compile-time flags have a higher maintenance than run-time flags.

3. 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.

4. 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!