macOS: bump to GCC 12 (for compilation and for use_dll)

The formulas are taken from commit aae5963a69896021466a037f500aa06bef405be4 of
the homebrew-core repository.

The gcc.rb formula was modified by adding “cellar: :any” to all bottles (see
https://github.com/Homebrew/brew/blob/master/docs/Bottles.md).

The preprocessor has been updated accordingly.