Fix detection of missing plugin for LTO objects.
    
    2019-08-16  Martin Liska  <mliska@suse.cz>
    
    	PR ld/24912
    	* elflink.c: Report error only for not relocatable.
    	* linker.c (_bfd_generic_link_add_one_symbol): Do not handle
    	here lto_slim_object as it's handled in caller.
    2019-08-16  Martin Liska  <mliska@suse.cz>
    
    	PR ld/24912
    	* object.cc (big_endian>::do_layout): Do not report error,
    	but only set a flag.
    	(big_endian>::do_add_symbols): Report error only for when
    	relocatable.