x86: Add _bfd_x86_elf_gc_mark_hook

x86: Add _bfd_x86_elf_gc_mark_hook

Since R_X86_64_GNU_VTINHERIT == R_386_GNU_VTINHERIT and
R_X86_64_GNU_VTENTRY == R_386_GNU_VTENTRY, we can share
_bfd_x86_elf_gc_mark_hook in elf32-i386.c and elf64-x86-64.c.

	* elf32-i386.c (elf_i386_gc_mark_hook): Removed.
	(elf_backend_gc_mark_hook): Likewise.
	* elf64-x86-64.c (elf_x86_64_gc_mark_hook): Likewise.
	(elf_backend_gc_mark_hook): Likewise.
	* elfxx-x86.c (_bfd_x86_elf_gc_mark_hook): New function.
	* elfxx-x86.h (_bfd_x86_elf_gc_mark_hook): New.
	(elf_backend_gc_mark_hook): Likewise.