gh-107082: Fix instruction size computation for ENTER_EXECUTOR

[gvanrossum]

If this fixes the big-endian buildbots it should also fixgh-107083.

• 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 @gvanrossum for commit 92b493d 🤖

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

[vstinner]

I tested manually the change on ppc64. I confirm that this change fix./python -m test -v test_capi on big endian machine.

I built Python with ./configure --with-pydebug && make -j32: it builds C code with gcc -Og, GCC 13.1.0 on Debian Sid.

Without the fix, I am able to reproduce the issue.

[vstinner]

LGTM. The change retrieves the original opcode stored in executors by insert_executor().

[vstinner]

Thanks for the fix.