PR21564 is my attempt to merge all changes needed to support macOS 11 and arm64.  To be honest I had lost track of what's needed.

With this the patch the tests pass on my DTK system. 

TODO:
- Verify builds on older macOS versions
- Maybe: support building using non-system libffi on macOS


PR 21583 is a separate attempt at doing the code changes that are needed to build with the latest SDK and deploy on macOS 10.9.  This patch is still in a fairly rough state, and needs more testing (I haven't done any testing on older platforms yet)

TODO:
- Test on macOS 10.9
- Tweak the build-installer script and possibly _osx_support.py: the minimal deployment target for arm64 should be macOS 11.0.
- Tweak the build-installer script to build an optimised version

TDB:
- Keep this as two separate PRs or merge them? 
  The latter makes testing easier, but review harder