LD/ELF: Unify STB_GNU_UNIQUE handling

Take STB_GNU_UNIQUE handling scattered across targets and gather it in
the generic ELF linker.  Update test suite infrastructure accordingly.

	bfd/
	* elf-s390-common.c (elf_s390_add_symbol_hook): Remove
	STB_GNU_UNIQUE handling.
	* elf32-arc.c (elf_arc_add_symbol_hook): Likewise.
	* elf32-arm.c (elf32_arm_add_symbol_hook): Likewise.
	* elf32-m68k.c (elf_m68k_add_symbol_hook): Likewise.
	* elf32-ppc.c (ppc_elf_add_symbol_hook): Likewise.
	* elf32-sparc.c (elf32_sparc_add_symbol_hook): Likewise.
	* elf64-ppc.c (ppc64_elf_add_symbol_hook): Likewise.
	* elf64-sparc.c (elf64_sparc_add_symbol_hook): Likewise.
	* elf64-x86-64.c (elf_x86_64_add_symbol_hook): Likewise.
	* elfxx-aarch64.c (_bfd_aarch64_elf_add_symbol_hook): Likewise.
	* elfxx-mips.c (_bfd_mips_elf_add_symbol_hook): Likewise.
	* elf32-i386.c (elf_i386_add_symbol_hook): Remove function.
	(elf_backend_add_symbol_hook): Remove macro.
	* elflink.c (elf_link_add_object_symbols): Set `has_gnu_symbols'
	for STB_GNU_UNIQUE symbols.

	binutils/
	* testsuite/lib/binutils-common.exp (supports_gnu_unique): New
	procedure.
	* testsuite/binutils-all/objcopy.exp: Use `supports_gnu_unique'
	with the `strip-10' test.

	ld/
	* testsuite/ld-unique/unique.exp: Use `is_elf_format' and
	`supports_gnu_unique' to qualify testing.