ELF: Pass bfd_link_info to merge_gnu_properties

Pass struct bfd_link_info * to merge_gnu_properties to give backend
control of how GNU properties are merged based on linker command line
options.

	* elf-bfd.h (elf_backend_data): Add struct bfd_link_info *
	to merge_gnu_properties.
	* elf-properties.c (elf_merge_gnu_properties): Add struct
	bfd_link_info * and pass it to merge_gnu_properties.
	(elf_merge_gnu_property_list): Add struct bfd_link_info *
	and pass it to elf_merge_gnu_properties.
	(_bfd_elf_link_setup_gnu_properties): Pass info to
	elf_merge_gnu_property_list.
	* elf32-i386.c (elf_i386_merge_gnu_properties): Add struct
	bfd_link_info *.
	* elf64-x86-64.c (elf_x86_64_merge_gnu_properties): Likewise.