only pass in a target arch to local builds when specified on the command line
    some arches notably x86, arm and sparc dont build for the base arch
    you end up with .i386 .arm or .sparc rpms when you really want something else
    
    we trust here that the host rpms macros are set correctly. which is
    redhat-rpm-config is installed they are and its a dep for us so we can trust it.