Add built in byte-swap identifiers.
    
    Silences these warnings:
    
    include/uapi/linux/swab.h:60:16: error: undefined identifier '__builtin_bswap32'
    include/uapi/linux/swab.h:60:33: error: not a function <noident>
    
    Signed-off-by: Jeff Layton <jlayton@redhat.com>