The problem with warning in elf_object_p

The problem with warning in elf_object_p

elfcode.h can emit three warnings in elf_object_p for various things,
"section extending past end of file", "corrupt string table index",
and "program header with invalid alignment".  The problem with doing
this is that the warning can be emitted for multiple possible targets
as each one is tried.  I was looking at a fuzzer testcase that had an
object file with 6144 program headers, 5316 of which had invalid
alignment.  It would be bad enough to get 5316 messages all the same,
but this object was contained in an archive and resulted in 4975776
repeats.

Some trimming can be done by not warning if the bfd is already marked
read_only, as is done for the "section extending past end of file"
warning, but that still results in an unacceptable number of
warnings for object files in archives.  Besides that, it is just wrong
to warn about a problem detected by a target elf_object_p other than
the one that actually matches.  At some point we might have more
target specific warnings.

So what to do?  One obvious solution is to remove the warnings.
Another is to poke any warning strings into the target xvec, emitting
them if that xvec is the final one chosen.  This also has the benefit
of solving the archive problem.  A warning when recursing into
_bfd_check_format for the first element of the archive (to find the
correct target for the archive) will still be on the xvec at the point
that target is chosen for the archive.  However, target xvecs are
read-only.  Thus the need for per_xvec_warn to logically extend
bfd_target with a writable field.  I've made per_xvec_warn one larger
than bfd_target_vector to provide one place for user code that makes
private copies of target xvecs.

	* elfcode.h (elf_swap_shdr_in, elf_object_p): Stash potential
	warnings in _bfd_per_xvec_warn location.
	* format.c (clear_warnmsg): New function.
	(bfd_check_format_matches): Call clear_warnmsg before trying
	a new xvec.  Print warnings for the successful non-archive
	match.
	* targets.c: Include libiberty.h.
	(_bfd_target_vector_entries): Use ARRAY_SIZE.
	(per_xvec_warn): New.
	(_bfd_per_xvec_warn): New function.
	* Makefile.am (LIBBFD_H_FILES): Add targets.c.
	* Makefile.in: Regenerate.
	* libbfd.h: Regenerate.