Take STT_GNU_IFUNC handling scattered across targets and gather it in
the generic ELF linker.

bfd/

	PR ld/23238
	* elf-s390-common.c (elf_s390_add_symbol_hook): Removed.
	* elf32-arc.c (elf_arc_add_symbol_hook): Likewise.
	(elf_backend_add_symbol_hook): Likewise.
	* elf32-m68k.c (elf_m68k_add_symbol_hook): Likewise.
	(elf_backend_add_symbol_hook): Likewise.
	* elf32-s390.c (elf_backend_add_symbol_hook): Likewise.
	* elf32-sparc.c (elf32_sparc_add_symbol_hook): Likewise.
	(elf_backend_add_symbol_hook): Likewise.
	* elf64-s390.c (elf_backend_add_symbol_hook): Likewise.
	* elfxx-aarch64.c (_bfd_aarch64_elf_add_symbol_hook): Likewise.
	* elfxx-aarch64.h (_bfd_aarch64_elf_add_symbol_hook): Likewise.
	(elf_backend_add_symbol_hook): Likewise.
	* elf32-arm.c (elf32_arm_add_symbol_hook): Remove STT_GNU_IFUNC
	handling.
	* elf32-ppc.c (ppc_elf_add_symbol_hook): Likewise.
	* elf64-ppc.c (ppc64_elf_add_symbol_hook): Likewise.
	* elf64-sparc.c (elf64_sparc_add_symbol_hook): Likewise.
	* elflink.c (elf_link_add_object_symbols): Set
	elf_gnu_symbol_ifunc for STT_GNU_IFUNC symbols.

ld/

	PR ld/23238
	* testsuite/ld-ifunc/ifunc-26.d: New file.
	* testsuite/ld-ifunc/ifunc-26.s: Likewise.
	* testsuite/ld-ifunc/ifunc.exp: Run *.d tests without a
	working compiler.