Check .gnu.warning section in shared library

	PR ld/15762
	* ld-elf/shared.exp (build_tests): Check .gnu.warning section
	in the libbarw.so library.
	* ld-elf/libbarw.rd: New.

	* lib/ld-lib.exp (run_cc_link_tests): Support checking on
	output with warning message.
	PR ld/15762
	PR ld/12761
	* elflink.c (elf_link_add_object_symbols): Correct test in
	last patch.  Remove unnecessary code.
	* elf64-ppc.c (struct ppc64_elf_obj_tdata): Replace opd_relocs
	with a union.
	(opd_entry_value): Cache .opd section contents.
	(ppc64_elf_relocate_section): Adjust.