Avoid the pointer hack in RCLASS_EXT

... because GCC 13 warns it.

```
In file included from class.c:24:
In function ‘RCLASS_SET_ALLOCATOR’,
    inlined from ‘class_alloc’ at class.c:251:5,
    inlined from ‘rb_module_s_alloc’ at class.c:1045:17:
internal/class.h:159:43: warning: array subscript 0 is outside array bounds of ‘rb_classext_t[0]’ {aka ‘struct rb_classext_struct[]’} [-Warray-bounds=]
  159 |     RCLASS_EXT(klass)->as.class.allocator = allocator;
      |                                           ^
```
https://rubyci.s3.amazonaws.com/arch/ruby-master/log/20231015T030003Z.log.html.gz

internal/class.h

@@ -85,7 +85,12 @@ struct RClass {
 // Assert that classes can be embedded in size_pools[2] (which has 160B slot size)
 STATIC_ASSERT(sizeof_rb_classext_t, sizeof(struct RClass) + sizeof(rb_classext_t) <= 4 * RVALUE_SIZE);
 
-#define RCLASS_EXT(c) ((rb_classext_t *)((char *)(c) + sizeof(struct RClass)))
+struct RClass_and_rb_classext_t {
+    struct RClass rclass;
+    rb_classext_t classext;
+};
+
+#define RCLASS_EXT(c) (&((struct RClass_and_rb_classext_t*)(c))->classext)
 #define RCLASS_CONST_TBL(c) (RCLASS_EXT(c)->const_tbl)
 #define RCLASS_M_TBL(c) (RCLASS(c)->m_tbl)
 #define RCLASS_IVPTR(c) (RCLASS_EXT(c)->iv_ptr)