Don't include coff/pe.h in coff-x86_64.c

Don't include coff/pe.h in coff-x86_64.c

This (and other) code from coffcode.h is broken for x86_64_coff_vec,
and has been ever since support was added in 2006 commit 99ad839030c1
Here, bfd_coff_aoutsz must match coff_swap_aouthdr_out otherwise we
end up writing garbage.

      /* Note that peicode.h fills in a PEAOUTHDR, not an AOUTHDR.
	 include/coff/pe.h sets AOUTSZ == sizeof (PEAOUTHDR)).  */
      char * buff;
      bfd_size_type amount = bfd_coff_aoutsz (abfd);

      buff = (char *) bfd_malloc (amount);
      if (buff == NULL)
	return false;

      coff_swap_aouthdr_out (abfd, & internal_a, buff);
      amount = bfd_bwrite (buff, amount, abfd);

We have removed support for --target=x86_64-coff, likely because it
never worked properly, but still produce coff-x86_64.o with
--enable-targets=all.  This means objcopy can recognize x86_64 COFF
files but will write garbage to the output file, a fact found by
fuzzers.  I suspect x86_64 COFF is still broken after this fix, and
mention of coff-x86_64.* should be removed from bfd/Makefile.am.

	* coff-x86_64.c: Don't include coff/pe.h.
	(COFF_WITH_pex64): Don't define here.
	* pe-x86_64.c: Include coff/pe.h and other headers.
	(PEI_HEADERS): Define.