I incline do not add skip_zacas stuffs (although skip_zabha is already there but that's fine), because that's different situation compare to the zaamo/zalrsc, zaamo/zalrsc should automatically append if a extension is available, which is new behavior and new extensions.
But zacas is only added when users explicitly add that in -march string unlike zaamo/zalrsc, so I am not sure if we need to check the binutils support and drop that if unsupported, My biggest concern is : should we do so for every new extension? I think we didn't do that so far, so we should
