Use nonstring attribute only on GCC 15

`__has_attribute` does not seem reliable.

Author: nobu

include/ruby/internal/attr/nonstring.h

@@ -23,7 +23,7 @@
 #include "ruby/internal/has/attribute.h"
 
 /** Wraps (or simulates) `__attribute__((nonstring))` */
-#if RBIMPL_HAS_ATTRIBUTE(nonstring)
+#if RBIMPL_COMPILER_SINCE(GCC, 15, 0, 0)
 # define RBIMPL_ATTR_NONSTRING() __attribute__((nonstring))
 #else
 # define RBIMPL_ATTR_NONSTRING() /* void */

symbol.c

@@ -172,7 +172,7 @@ rb_id_attrset(ID id)
 
     /* make new symbol and ID */
     if (!(str = lookup_id_str(id))) {
-        static const char id_types[][8] = {
+        RBIMPL_ATTR_NONSTRING() static const char id_types[][8] = {
             "local",
             "instance",
             "invalid",