macOS: Improve deployment target management for universal binaries

[kintel]

We build universal binaries (arm64 + x86_64) for macOS. However, the two architectures requires different minimum macOS version to run: arm64 is only available since macOS 11, and our x86_64 build can still run on macOS 10.13 or so, or even lower if built carefully on older compilers.

Some things should be cleaned up:

• Our binary checker (https://github.com/openscad/openscad/blob/master/scripts/macosx-sanity-check.py) doesn't have a good arm64 check.
• Our CMake file only sets the minimum target for x86_64 (unspecified really, but it seems to apply to x86_64)
• Dependencies built with our script (https://github.com/openscad/openscad/blob/master/scripts/macosx-build-dependencies.sh) only considers x86_64 and arm64 gets some automatic minimum version applied