Here is a patch that implements _Py_memset_s() according to C11 with a fallback to memset_s().

My linker fu seems to be weak. I had to use _Py_memset_s() in random.c otherwise the function is removed from the static Python binary. I double-checked with readelf -Ws: the function is defined in the archive file and is available in the shared library. But the linker removes it from the static binary so shared libraries fail to load.