Fix override of common symbols for a.out
    
    The default should be bfd_link_common_skip_none, as in the original
    patch at https://sourceware.org/ml/binutils/2002-07/msg00717.html
    
    	* aoutx.h (aout_link_check_ar_symbols): Remove default and handle
    	bfd_link_common_skip_none in switch.