add a boundary to imp.exec_builtin

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/ImpModuleBuiltins.java

@@ -357,6 +357,7 @@ private PythonModule getBuiltinModule(String name) {
     @GenerateNodeFactory
     public abstract static class ExecBuiltin extends PythonBuiltinNode {
         @Specialization
+        @TruffleBoundary
         public Object exec(PythonModule pythonModule) {
             final Python3Core core = getCore();
             if (!ImageInfo.inImageBuildtimeCode()) {