Fix detection of missing plugin for LTO objects.

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.