Try to fix compilation on m68k

Compilation is failing on m68k-linux with:

```
./include/ruby/internal/static_assert.h:51:46: error: static assertion failed: "sizeof_method_def: offsetof(rb_method_definition_t, body)==8"
   51 | # define RBIMPL_STATIC_ASSERT0 __extension__ _Static_assert
      |                                              ^~~~~~~~~~~~~~
./include/ruby/internal/static_assert.h:70:5: note: in expansion of macro 'RBIMPL_STATIC_ASSERT0'
   70 |     RBIMPL_STATIC_ASSERT0(expr, # name ": " # expr)
      |     ^~~~~~~~~~~~~~~~~~~~~
./internal/static_assert.h:13:24: note: in expansion of macro 'RBIMPL_STATIC_ASSERT'
   13 | # define STATIC_ASSERT RBIMPL_STATIC_ASSERT
      |                        ^~~~~~~~~~~~~~~~~~~~
./method.h:203:1: note: in expansion of macro 'STATIC_ASSERT'
  203 | STATIC_ASSERT(sizeof_method_def, offsetof(rb_method_definition_t, body)==8);
      | ^~~~~~~~~~~~~
```

method.h

@@ -200,7 +200,7 @@ struct rb_method_definition_struct {
 struct rb_id_table;
 
 typedef struct rb_method_definition_struct rb_method_definition_t;
-STATIC_ASSERT(sizeof_method_def, offsetof(rb_method_definition_t, body)==8);
+STATIC_ASSERT(sizeof_method_def, offsetof(rb_method_definition_t, body) <= 8);
 
 #define UNDEFINED_METHOD_ENTRY_P(me) (!(me) || !(me)->def || (me)->def->type == VM_METHOD_TYPE_UNDEF)
 #define UNDEFINED_REFINED_METHOD_P(def) \