[3.8] bpo-41100: Support macOS 11 Big Sur and Apple Silicon Macs (#25806)

* bpo-41100: Support macOS 11 and Apple Silicon on Python 3.8

This is a partial backport of bpo-41100 changes `e8b1c038` and `96d906b1` 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 `7e729978` to fix build errors with external `libffi`.

* bpo-41116: Ensure system supplied libraries are found on macOS 11 (GH-23301) (GH-23455)

On macOS system provided libraries are in a shared library cache
and not at their usual location. This PR teaches distutils to search
in the SDK, even if there was no "-sysroot" argument in
the compiler flags.
(cherry picked from commit 404a719b)

* bpo-42504: fix for MACOSX_DEPLOYMENT_TARGET=11 (GH-23556)

macOS releases numbering has changed as of macOS 11 Big Sur.  Previously, major releases were of the form 10.x, 10.x+1, 10.x+2, etc; as of Big Sur, they are now x, x+1, etc, so, for example, 10.15, 10.15.1, ..., 10.15.7, 11, 11.0.1, 11.1, ..., 12, 12.1, etc. Allow Python to build with single-digit deployment target values. Patch provided by FX Coudert.
(cherry picked from commit 5291639e)

* bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string (GH-24341) (GH-24410)

* bpo-42504: Ensure that get_config_var('MACOSX_DEPLOYMENT_TARGET') is a string
(cherry picked from commit 49926cf2)

Co-authored-by: Ronald Oussoren <ronaldoussoren@mac.com>
Co-authored-by: FX Coudert <fxcoudert@gmail.com>
Co-authored-by: Max Bélanger <aeromax@gmail.com>