[3.9] bpo-41100: Support macOS 11 and Apple Silicon (GH-22855) (GH-23295)

[3.9] bpo-41100: Support macOS 11 and Apple Silicon (GH-22855) (GH-23295)

* [3.9] bpo-41100: Support macOS 11 and Apple Silicon (GH-22855)

Co-authored-by:  Lawrence D’Anna <lawrence_danna@apple.com>
* Add support for macOS 11 and Apple Silicon (aka arm64)

  As a side effect of this work use the system copy of libffi on macOS, and remove the vendored copy

* Support building on recent versions of macOS while deploying to older versions

  This allows building installers on macOS 11 while still supporting macOS 10.9..
(cherry picked from commit 41761933c1c30bb6003b65eef1ba23a83db4eae4)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>
* Back port of changes to _decimal to support arm64

* temp_dir is in test.support in 3.9