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.

Dockerfile.dev

@@ -37,7 +37,7 @@ COPY install-wrappers.sh .
 RUN ./install-wrappers.sh $TOOLCHAIN_PREFIX
 
 # Build MinGW-w64
-COPY build-mingw-w64.sh .
+COPY build-mingw-w64.sh build-mingw-w64-widl.sh ./
 RUN ./build-mingw-w64.sh $TOOLCHAIN_PREFIX
 
 # Build compiler-rt