Fix optional/gc_specs.rb and use LL2NUM instead of LONG2NU

INT64_MAX might be long long int and there might be overflow with LONG2NU, e.g. in windows job:
```
gc_spec.c: In function 'Init_gc_spec':
gc_spec.c:108:58: warning: overflow in conversion from 'long long int' to 'long int' changes value from '9223372036854775807' to '-1' [-Woverflow]
  108 |   registered_before_rb_global_variable_bignum = LONG2NUM(INT64_MAX);
      |                                                          ^~~~~~~~~
gc_spec.c:113:57: warning: overflow in conversion from 'long long int' to 'long int' changes value from '9223372036854775807' to '-1' [-Woverflow]
  113 |   registered_after_rb_global_variable_bignum = LONG2NUM(INT64_MAX);
      |                                                         ^~~~~~~~~
```

optional/capi/ext/gc_spec.c

@@ -105,12 +105,12 @@ void Init_gc_spec(void) {
   registered_before_rb_gc_register_address = rb_str_new_cstr("registered before rb_gc_register_address()");
 
   registered_before_rb_global_variable_string = rb_str_new_cstr("registered before rb_global_variable()");
-  registered_before_rb_global_variable_bignum = LONG2NUM(INT64_MAX);
+  registered_before_rb_global_variable_bignum = LL2NUM(INT64_MAX);
   registered_before_rb_global_variable_float = DBL2NUM(3.14);
 
   registered_after_rb_global_variable_string = rb_str_new_cstr("registered after rb_global_variable()");
   rb_global_variable(&registered_after_rb_global_variable_string);
-  registered_after_rb_global_variable_bignum = LONG2NUM(INT64_MAX);
+  registered_after_rb_global_variable_bignum = LL2NUM(INT64_MAX);
   rb_global_variable(&registered_after_rb_global_variable_bignum);
   registered_after_rb_global_variable_float = DBL2NUM(6.28);
   rb_global_variable(&registered_after_rb_global_variable_float);