gfortran 12.2-darwin-r0 universal binary

This build is a fully universal binary of Iain Sandoe's release of GNU Fortran 12.2-darwin-r0 for macOS 11 (Big Sur) and higher.

It installs into /opt/gfortran and /opt/gfortran/bin must be on the PATH for it to work properly. The /opt/gfortran/SDK symlink must point to the desired SDKROOT. E.g., you can update it with

ln -sfn $(xcrun --show-sdk-path) /opt/gfortran/SDK

Fully universal means that it runs on both arm64 and x86_64 architectures and it can produce code for both arm64 and x86_64 (technically, it consists of two native compilers and two cross-compilers - see build/README for details).

Note: this binary includes gcc as well, which is not strictly necessary, but the only driver-driver gfortran (which dispatches to the corresponding *-apple-darwin20.0-gfortran driver) is on the PATH. It does not support in-flight lipo (i.e. multiple -arch switches at once), so either use the experimental driver shell script or compile for each architecture separately. Other tools don't have driver-drivers, but can be used via the full <arch>-apple-darwin20.0-<tool> name if desired.