standalone JPS distribution: extract 'java-runtime' module to a separate JAR

This is needed to ensure that the whole jps-builders.jar won't be added to classpath when javac process is forked. Otherwise the classpath will include *.class files which aren't compatible with Java 6, and it'll lead to exceptions if javac from JDK 1.6 is used for compilation.

build/scripts/layouts.gant

@@ -44,6 +44,8 @@ def layoutJps(String home, String targetDir, String buildNumber, Closure additio
         module("javac-ref-scanner-8")
         module("jps-builders")
         module("jps-standalone-builder")
+      }
+      jar("idea_rt.jar") {
         module("java-runtime")
       }
       jar("jps-builders-6.jar") {