MVK: clean the environment on repeated compilations

buildMobileVLCKit.sh

@@ -625,6 +625,8 @@ buildMobileKit() {
         export STRIP="xcrun strip"
         export CPP="xcrun cc -E"
         export CXXCPP="xcrun c++ -E"
+        export AS="xcrun as"
+        export CCAS=""
 
         if [ "$TVOS" = "yes" ]; then
             if [ "$PLATFORM" = "iphonesimulator" ]; then