Build: Use javadocJar helper to specify artifact explicitly

Creating javadocJar task for every project produces lots of unnecessary
tasks, some project don't even have code. Jar task without outDir
property set fails idea import with gradle 5.0+

libraries/stdlib/js/build.gradle

@@ -345,9 +345,10 @@ artifacts {
     runtime mergedJar
     archives mergedJar
     archives sourcesJar
-    archives javadocJar
 }
 
+javadocJar()
+
 task distJs(type: Copy) {
     from(compileJs)
     into "$distDir/js"