Backporting provisions and clang support for Apple Silicon (arm64)

- provisions for Apple Silicon package
(backported fe8aaf44)

- ModelTree::findGccOnMacOS() now returns a std::filesystem::path
(backported bae04fa8)

- use_dll: under Windows, append MinGW location to the PATH variable only once [By the way, move the macOS+Octave environment variable initializations to the
same place, for consistency.]
(backported bff80c0e)

- macOS: use clang if GCC is not available for use_dll [Related to Dynare/dynare#1893 and Dynare/dynare#1894]
(backported 958b0a38)