RFC Final Comment Period: Establish default build flags for Fortran

An RFC has now entered Final Comment Period. In 14 days, discussion will end and the proposal will either be accepted, rejected or withdrawn: https://gitlab.archlinux.org/archlinux/rfcs/-/merge_requests/54 Please visit the above link for discussion. Summary: This RFC establishes default build flags (`FFLAGS`, `FCFLAGS`, `DEBUG_FFLAGS`) for compiling Fortran code in Arch Linux packages, aligning them with the security hardening standards defined for C.