Re: Add section caches to coff_data_type

Another thing, section target_index is renumbered in
coff_compute_section_file_positions and _bfd_xcoff_bfd_final_link.  I
don't know that there is currently any way that the output bfd
section_by_target_index could be populated before this point but
clear them out so no one need worry about it.

	* coffcode.h (coff_compute_section_file_positions): Clear
	section_by_target_index hash table when changing target_index.
	(_bfd_xcoff_bfd_final_link): Likewise.