atlas uses cpu optimizing flags based on arch: ppc = G4, ppc64 = G5; the former needs not be correct

[barracuda156]

} elseif {${build_arch} eq "ppc"} {

    # PPC
    set gcc_flags           "-O3 -mcpu=7400 -mtune=7400\
                            -fno-schedule-insns -fschedule-insns2 -fPIC \
                            -force_cpusubtype_ALL"
    set clang_flags         "-O3 -arch ppc -fPIC"

    set gvct_flag           "-maltivec -mabi=altivec"
    set cvct_flag           "-maltivec"
    
    configure.args-append   -A 4

    # On Leopard, we may warn the user that ppc 
    # is not as efficient as ppc64
    pre-fetch {
        if {${native_arch} eq "ppc64"} {

                ui_msg "Compiling for ppc on a G5 machine.\
                        This could lead to reduced performance."
        }
    }

} else {

    # PPC64
    set gcc_flags           "-mcpu=970 -mtune=970 -mvrsave -mpowerpc64\
                            -fno-schedule-insns -fschedule-insns2 -fPIC \
                            -force_cpusubtype_ALL"
    # Clang has no PPC64 target so compile for PPC
    set clang_flags         "-O3 -arch ppc -fPIC"

    set gvct_flag           "-maltivec -mabi=altivec"
    set cvct_flag           "-maltivec"
}

While ppc64 requires G5, ppc can be either. It is desirable to have variants for both, so that the user can choose, or use -mtune=native (if old GCC supports that). What we have now gonna make suboptimal optimization on G5 when build is for ppc32 (which is most cases, as long as Macports goes).