macros: Pass -Ccodegen-units=1 in RUSTFLAGS
    
    This has good impact on performance and binary size at cost of
    compilation time. As nice side effect, it fixes bug which breaks
    compilation of binaries in Fedora Rawhide.
    
    For example, ffsend binary went from 107M to 70M.
    
    References: https://bugzilla.redhat.com/show_bug.cgi?id=1701339
    Signed-off-by: Igor Gnatenko <ignatenkobrain@fedoraproject.org>