gh-107082: Fix _Py_CODEUNIT struct for big endian

[vstinner]

• Issue: test_capi.test_basic_loop(): _PyInstruction_GetLength() assertion error on s390x Fedora Clang 3.x buildbot #107082

[bedevere-bot]

🤖 New build scheduled with the buildbot fleet by @vstinner for commit 42c00bc 🤖

If you want to schedule another build, you need to add the 🔨 test-with-buildbots label again.

[vstinner]

buildbot/PPC64 Fedora PR failed to build:

_bootstrap_python: ../Python/generated_cases.c.h:3765: _PyEval_EvalFrameDefault: Assertion `((void)0, ((_Py_CODEUNIT *)((PyCodeObject *)shim->f_executable)->co_code_adaptive))[1].op.code == 16' failed. 

[vstinner]

This change is just wrong :-)