Keeping track of rs6000-coff archive element pointers

rs6000-coff archives use a linked list of file offsets, where each
element points to the next element.  The idea is to allow updating of
large archives quickly without rewriting the whole archive.  (binutils
ar does not do this.)  Unfortunately this is an easy target for
fuzzers to create an archive that will cause ar or any other tool
processing archives to hang.  I'd implemented guards against pointing
back to the previous element, but of course that didn't last long.

So this patch implements a scheme to keep track of file offset ranges
used by elements as _bfd_read_ar_hdr is called for each element.  See
the add_range function comment.  I needed a place to stash the list,
so chose the obvious artdata.tdata backend extension to archive's
tdata, already used by xcoff.  That involved a little cleanup, because
while it would be possible to continue using different artdata.tdata
for the big and small archives, it's nicer to use a union.

If anyone is concerned this list of element ranges might grow large
and thus significantly slow down the tools, adjacent ranges are
merged.  In fact something like "ar t" will only ever have one range
on xcoff archives generated by binutils/ar.  I agree there might still
be a problem with ld random element access via the armap.

include/
	* coff/xcoff.h (SIZEOF_AR_FILE_HDR): Use sizeof.
	(SIZEOF_AR_FILE_HDR_BIG, SIZEOF_AR_HDR, SIZEOF_AR_HDR_BIG): Likewise.
	(struct ar_ranges, struct xcoff_artdata): New.
	(x_artdata): Define.
	(xcoff_big_format_p): Rewrite.
	(xcoff_ardata, xcoff_ardata_big): Delete.
bfd/
	* coff-rs6000.c: Replace uses of xcoff_ardata and
	xcoff_ardata_big throughout file.
	(_bfd_xcoff_archive_p): Adjust artdata.tdata allocation.
	(add_range): New function.
	(_bfd_xcoff_read_ar_hdr): Use it here.  Fix memory leak.
	(_bfd_xcoff_openr_next_archived_file): Remove old sanity
	checks.  Set up range for header.
	(xcoff_write_archive_contents_old): Make the temporary
	artdata.tdata used here to pass info down to
	_bfd_compute_and_write_armap a struct xcoff_artdata.
	(xcoff_write_archive_contents_big): Likewise.
	* coff64-rs6000.c: Replace uses of xcoff_ardata and
	xcoff_ardata_big throughout file.
	(xcoff64_archive_p): Adjust artdata.tdata allocation.

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