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/libraries/zlib/default.nix

@@ -53,7 +53,7 @@ stdenv.mkDerivation rec {
 
   crossAttrs = {
     dontStrip = static;
-    dontSetConfigureCross = true;
+    configurePlatforms = [];
   } // stdenv.lib.optionalAttrs (stdenv.cross.libc == "msvcrt") {
     installFlags = [
       "BINARY_PATH=$(out)/bin"