Add mingw64 crossbuild script, patch by Zack Middleton

cross-make-mingw64.sh

@@ -0,0 +1,10 @@
+#!/bin/sh
+
+export CC=amd64-mingw32msvc-gcc
+export WINDRES=amd64-mingw32msvc-windres
+export PLATFORM=mingw32
+if [ !$ARCH ]
+then
+export ARCH=x86_64
+fi
+exec make $*