building on arm64 Ubuntu 22.04 gives thousands of warnings

Building on arm64 ubuntu 22.04 gives thousands of warnings. Far too many to list here, but as an example:

```
/tmp/tmp.gBSBQdyNGN/Python-3.6.1/Objects/bytesobject.c:2481:5: note: in expansion of macro ‘BYTES_RSPLIT_METHODDEF’
2481 | BYTES_RSPLIT_METHODDEF
| ^~~~~~~~~~~~~~~~~~~~~~
/tmp/tmp.gBSBQdyNGN/Python-3.6.1/Objects/clinic/bytesobject.c.h:20:15: warning: cast between incompatible function types from ‘PyObject * (*)(PyBytesObject *, PyObject **, Py_ssize_t, PyObject *)’ {aka ‘struct _object * (*)(PyBytesObject *, struct _object **, long int, struct _object *)’} to ‘PyObject * (*)(PyObject *, PyObject *)’ {aka ‘struct _object * (*)(struct _object *, struct _object *)’} [-Wcast-function-type]
20 | {"split", (PyCFunction)bytes_split, METH_FASTCALL, bytes_split__doc__},
```

```
/tmp/tmp.gBSBQdyNGN/Python-3.6.1/Objects/stringlib/codecs.h:332:19: warning: this statement may fall through [-Wimplicit-fallthrough=]
332 | p += (endpos - startpos);
```

arm64 Ubuntu 22.04 で build を再現し、警告一式を記録します。まず Objects/bytesobject.c、Objects/clinic/bytesobject.c.h、Objects/stringlib/codecs.h の例から始め、次に警告が想定されたものか、ビルドの問題を示しているかを判断します。警告の挙動を理解し、適切な修正または文書化された制限事項を検証できれば完了です。

索引モデルが issue の本文から書いたものです。