Add boundary for System.nanoTime

Author: bjfish

src/main/java/org/truffleruby/core/ProcessNodes.java

@@ -27,6 +27,7 @@ public abstract class ProcessNodes {
     @Primitive(name = "process_time_nanotime")
     public abstract static class ProcessTimeNanoTimeNode extends PrimitiveArrayArgumentsNode {
 
+        @TruffleBoundary
         @Specialization
         protected long nanoTime() {
             return System.nanoTime();