[3.8] bpo-41100: Support macOS 11 and Apple Silicon by maxbelanger · Pull Request #25274 · python/cpython

This is a partial backport of bpo-41100 changes `e8b1c038b14b5fc8120aab62c9bf5fb840274cb6` and `96d906b144e6e6aa96c5ffebecbcc5d38034bbda` for Python 3.8. We introduce the ability to build Python from source for `arm64` on macOS, but we do not make a promise of support. This allows us to omit support for Universal2 binaries as well as weak-linking of symbols from the macOS SDK based on the deployment target, which are larger changes much more difficult to merge.

This also includes a backport of subsequent bpo-42688 change `7e729978fa08a360cbf936dc215ba7dd25a06a08` to fix build errors with external `libffi`.