Sulong: Override RootNode.isInternal() to return a PE-constant value

This fixes a performance warning due to a non PE-constant default implementation of
isInternal() in Truffle.

Author: fmorcos

sulong/projects/com.oracle.truffle.llvm.runtime/src/com/oracle/truffle/llvm/runtime/nodes/intrinsics/multithreading/LLVMPThreadStart.java

@@ -129,6 +129,11 @@ public static LLVMPThreadFunctionRootNode create(LLVMLanguage language, NodeFact
             return new LLVMPThreadFunctionRootNode(language, descriptor, functionSlot, argSlot, nodeFactory);
         }
 
+        @Override
+        public boolean isInternal() {
+            return false;
+        }
+
         @Override
         public Object execute(VirtualFrame frame) {
             if (ctxRef == null) {