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/common/build.gradle

@@ -103,9 +103,10 @@ configurations {
 artifacts {
     archives sourcesJar
     sources sourcesJar
-    archives javadocJar
 }
 
+javadocJar()
+
 // TODO: call the "dist" task instead, once we need to publish kotlin-stdlib-common.jar with the compiler distribution
 task distCommon(type: Copy) {
     from(jar)