bpo-45434: Only exclude <stdlib.h> in Python 3.11 limited C API (GH-29027)

The Python 3.11 limited C API no longer includes stdlib.h, stdio.h,
string.h and errno.h.

* Exclude Py_MEMCPY() from Python 3.11 limited C API.
* xxlimited C extension is now built with Python 3.11 limited C API.