gh-156933: Widen narrow integer results in ctypes callbacks

[lazerg]

_CallPythonObject() only wrote restype->size bytes into the closure's result buffer, leaving the unused high-order bits of the ffi_arg-sized register untouched. libffi's ffi_prep_closure_loc() documents that integral types narrower than a machine register must be widened to fill it, sign-extending signed types. On architectures that always read the full register for narrow return values (s390x), this leaves garbage in the high bits, which broke libclang callbacks used by cindex.py.

The fix widens narrow integer results into a register-sized buffer before writing them back, sign- or zero-extending depending on the type, replacing the old big-endian-only pointer offset that didn't actually widen anything.

Fixes #156933.

• Issue: _ctypes fails to extend the return value per libffi contract #156933

[python-cla-bot]

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[lazerg]

This failure is a 10 minute timeout in test_subprocess's test_check on the macOS Intel runner, unrelated to this change (which only touches _ctypes/callbacks.c).