x86: Add _bfd_x86_elf_get_synthetic_symtab

x86: Add _bfd_x86_elf_get_synthetic_symtab

Move the common codes in elf_i386_get_synthetic_symtab and
elf_x86_64_get_synthetic_symtab to _bfd_x86_elf_get_synthetic_symtab.

	* elf32-i386.c (elf_i386_plt_type): Removed.
	(elf_i386_plt): Likewise.
	(elf_i386_get_synthetic_symtab): Updated.   Call
	_bfd_x86_elf_get_synthetic_symtab.
	* elf64-x86-64.c (elf_x86_64_plt_type): Removed.
	(elf_x86_64_plt): Likewise.
	(elf_x86_64_get_synthetic_symtab): Updated.  Call
	_bfd_x86_elf_get_synthetic_symtab.
	* elfxx-x86.c (elf_i386_get_plt_got_vma): New function.
	(elf_x86_64_get_plt_got_vma): Likewise.
	(elf_i386_valid_plt_reloc_p): Likewise.
	(elf_x86_64_valid_plt_reloc_p): Likewise.
	(_bfd_x86_elf_get_synthetic_symtab): Likewise.
	* elfxx-x86.h (elf_x86_plt_type): New.
	(elf_x86_plt): Likewise.
	(_bfd_x86_elf_get_synthetic_symtab): Likewise.