Change _bfd_elf_link_setup_gnu_properties to bfd *

Change _bfd_elf_link_setup_gnu_properties to bfd *

Change setup_gnu_properties to return the first relocatable ELF input
with GNU properties so that a backend can make decision based on GNU
properties.

	* elf-bfd.h (elf_backend_data): Change setup_gnu_properties
	to return bfd *.
	(_bfd_elf_link_setup_gnu_properties): Return bfd *.
	* elf-properties.c (_bfd_elf_link_setup_gnu_properties): Return
	the first relocatable ELF input with GNU properties.