Add section caches to coff_data_type

Add section caches to coff_data_type

  * libcoff-in.h (struct coff_tdata): Add section_by_index and section_by_target_index hash tables.
  * libcoff.h: Regenerate.
  * coffcode.h (htab_hash_section_index): New function. (htab_eq_section_index): New function. (htab_hash_section_target_index): New function. (htab_eq_section_target_index): New function. (coff_mkobject_hool): Create the hash tables.
  * peicode.h: Add the same new functions. (pe_mkobject_hook): Create the hash tables.
  * coff-x86_64.c (coff_amd64_rtype_to_howto): Use the new tables to speed up lookups.
  * coffgen.c (coff_section_from_bfd_index): Likewise. (_bfd_coff_close_and_cleanup): Delete the hash tables.