Fix loading system libraries with ctypes on macOS Big Sur.
On Big Sur, there is no longer a separate dylib file for each system library. Instead, they have been combined into a single shared cache file. As a result, it no longer suffices to check file existence to determine whether a system library is present. Fix this by calling _dyld_shared_cache_contains_path to check the dyld shared cache as well.
The changes under lib-python/ are copied from https://github.com/python/cpython/pull/22855.
The _dyld_shared_cache_contains_path function is exposed to the ctypes python code using CFFI, similar to what is done in other modules, such as lib_pypy/_sha3.
Fixes issue #3314 (closed).