Split out building widl to a separate shell script

When cross compiling a llvm-mingw toolchain, we need to build widl
for the intended host environment as well.

build-mingw-w64.sh

@@ -63,12 +63,5 @@ for arch in $ARCHS; do
 done
 cd ..
 
-cd mingw-w64-tools/widl
-for arch in $ARCHS; do
-    mkdir -p build-$arch
-    cd build-$arch
-    ../configure --prefix=$PREFIX --target=$arch-w64-mingw32 INSTALL="install -s"
-    make -j$CORES
-    make install
-    cd ..
-done
+cd ..
+./build-mingw-w64-widl.sh "$PREFIX"