Provisions for building both x86_64 and arm64 pkg on an Apple Silicon mac

MATLAB has finally released an Apple Silicon version (R2022a beta and R2022b beta), so we can now start providing a native Apple pkg. With this commit (and the related preprocessor commit preprocessor!61 (merged)), I can compile Dynare from source without using Rosetta 2. The testsuite runs through except for tests that require additional MATLAB toolboxes (e.g. Optimization) as this is not released yet in the ARM Beta versions. Changes to the build script will be in another merge request once we have installed the new Apple Silicon runner.

To Do:

• detect new architectures maca64 and mexmaca64
• check whether one needs to deal with hard-coded /usr/local instead of /opt/homebrew paths
• create both x86_64 and arm64 packages on @wmutschl M1s.
• test x86_64 pkg on @wmutschl M1 using MATLAB R2022a under Rosetta2
• test arm64 pkg on @wmutschl M1 using MATLAB R2022a Beta for Apple Silicon
• test x86_64 pkg on clean-install (virtual) Intel mac (created with quickemu on Linux)
• test arm64 pkg on clean-install (virtual) Apple Silicon mac (created with parallels)
• merge preprocessor!61 (merged)
• file a bug with MATLAB R2022a BETA that /Applications/MATLAB_R2022a_Beta.app/bin/mexext needs some small changes to detect mexmaca64.
• update the manual