cross stdenv adaptor: Support --host --build --target across the board

Packages get --host and --target by default, but can explicitly request
any subset to be passed as needed. See docs for more info.

rustc: Avoid hash breakage by using the old (ignored)
dontSetConfigureCross when not cross building

pkgs/development/compilers/ghc/head.nix

@@ -114,7 +114,7 @@ in stdenv.mkDerivation (rec {
 
   buildInputs = commonBuildInputs ++ [ stdenv.ccCross stdenv.binutils ];
 
-  dontSetConfigureCross = true;
+  configurePlatforms = [];
 
   passthru = {
     inherit bootPkgs cross;