Add _bfd_elf_ifunc_get_synthetic_symtab

Add _bfd_elf_ifunc_get_synthetic_symtab

In i386 and x86-64 binaries with ifunc, relocations against .got.plt
section may not be in the same order as entries in PLT section.  This
patch adds _bfd_elf_ifunc_get_synthetic_symtab.  It takes a function
pointer which returns an array of PLT entry symbol values.  It calls
the function pointer to get the PLT entry symbol value array indexed
by relocation index, instead of calling plt_sym_val on each relocation
index.

	PR binutils/17677
	* elf-bfd.h (_bfd_elf_ifunc_get_synthetic_symtab): New prototype.
	* elf-ifunc.c (_bfd_elf_ifunc_get_synthetic_symtab): New
	function.
	* elf32-i386.c (elf_i386_plt_sym_val): Removed.
	(elf_backend_plt_sym_val): Likewise.
	(elf_i386_get_plt_sym_val): New.
	(elf_i386_get_synthetic_symtab): Likewise.
	(bfd_elf32_get_synthetic_symtab): Likewise.
	* elf64-x86-64.c (elf_x86_64_plt_sym_val): Removed.
	(elf_x86_64_plt_sym_val_offset_plt_bnd): Likewise.
	(elf_backend_plt_sym_val): Likewise.
	(elf_x86_64_get_plt_sym_val): New.
	(elf_x86_64_get_synthetic_symtab): Use
	_bfd_elf_ifunc_get_synthetic_symtab.
	(bfd_elf64_get_synthetic_symtab): Don't undefine for NaCl.