haskellPackages: use static-enabled 'gmp' to enable statically linked Haskell binaries

Pass '-optl-static -optl-pthread' to GHC to enable static linking.

Fixes <https://github.com/NixOS/nixpkgs/issues/3193>. Further details can be
found at <http://permalink.gmane.org/gmane.linux.distributions.nixos/13526>.
ghc: update version 7.8.2 to 7.8.3
haskell-packages.nix: cosmetic
Switch default Haskell compiler to GHC 7.8.3.