Add convenience phony targets for system modules

OpenJDK9 system modules are not build modules, which means they
can't be built with m core-system-modules.  Add a phony target.

Test: m core-system-modules
Change-Id: If6d512ff7a009b49743fb25cbb566935ec1c0153

java/system_modules.go

@@ -138,6 +138,7 @@ func (system *SystemModules) AndroidMk() android.AndroidMkData {
 				fmt.Fprintln(w)
 				fmt.Fprintln(w, makevar, ":=", system.outputFile.String())
 				fmt.Fprintln(w, ".KATI_READONLY", ":=", makevar)
+				fmt.Fprintln(w, name+":", "$("+makevar+")")
 			}
 		},
 	}