Move common symbol check after bed->common_definition
    
    bfd/
    
    	* elflink.c (elf_link_add_object_symbols): Move common symbol
    	check after bed->common_definition.
    
    ld/
    
    	* testsuite/ld-elf/pr21903.s (start): Removed.
    	(_start): Likewise.
    	(__start): Likewise.
    	(main): Likewise.
    	(bar): New.