StringBuilder.append() and String.valueOf() must not be in PE code

* Found by SVM's check.

Author: eregon

src/main/java/org/truffleruby/core/string/StringNodes.java

@@ -2166,7 +2166,7 @@ protected RubyString undumpNonAsciiCompatible(RubyString string) {
             throw new RaiseException(
                     getContext(),
                     getContext().getCoreExceptions().encodingCompatibilityError(
-                            "ASCII incompatible encoding: " + string.rope.encoding,
+                            Utils.concat("ASCII incompatible encoding: ", string.rope.encoding),
                             this));
         }