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/ffmpeg/generic.nix

@@ -187,7 +187,7 @@ stdenv.mkDerivation rec {
       fi
     '';
   in {
-    dontSetConfigureCross = true;
+    configurePlatforms = [];
     configureFlags = configureFlags ++ [
       "--cross-prefix=${stdenv.cross.config}-"
       "--enable-cross-compile"