parse.y: `__ENCODING__` should be determined in run-time; fix #5552

It used to be compiled to the static string in the compiler. But the
encoding status actually depends on the runtime configuration. A new
method `Kernel#__ENCODING__` is introduced to implement the feature.

mrbgems/mruby-compiler/core/parse.y

@@ -3587,12 +3587,7 @@ var_ref         : variable
                     }
                 | keyword__ENCODING__
                     {
-#ifdef MRB_UTF8_STRING
-                      const char *enc = "UTF-8";
-#else
-                      const char *enc = "ASCII-8BIT";
-#endif
-                      $$ = new_str(p, enc, strlen(enc));
+                      $$ = new_fcall(p, MRB_SYM_2(p->mrb, __ENCODING__), 0);
                     }
                 ;
 

src/kernel.c

@@ -612,6 +612,17 @@ mrb_obj_ceqq(mrb_state *mrb, mrb_value self)
   return mrb_false_value();
 }
 
+static mrb_value
+mrb_encoding(mrb_state *mrb, mrb_value self)
+{
+  mrb_get_args(mrb, "");
+#ifdef MRB_UTF8_STRING
+  return mrb_str_new_lit(mrb, "UTF-8");
+#else
+  return mrb_str_new_lit(mrb, "ASCII-8BIT");
+#endif
+}
+
 mrb_value mrb_obj_equal_m(mrb_state *mrb, mrb_value);
 
 void
@@ -652,6 +663,7 @@ mrb_init_kernel(mrb_state *mrb)
   mrb_define_method(mrb, krn, "to_s",                       mrb_any_to_s,                    MRB_ARGS_NONE());    /* 15.3.1.3.46 */
   mrb_define_method(mrb, krn, "__case_eqq",                 mrb_obj_ceqq,                    MRB_ARGS_REQ(1));    /* internal */
   mrb_define_method(mrb, krn, "__to_int",                   mrb_to_integer,                  MRB_ARGS_NONE());    /* internal */
+  mrb_define_method(mrb, krn, "__ENCODING__",               mrb_encoding,                    MRB_ARGS_NONE());
 
   mrb_include_module(mrb, mrb->object_class, mrb->kernel_module);
 }