libctf: warn about information loss because of unreleased format changes

In the last cycle there have been various changes that have replaced
parts of the CTF format with other parts without format
compatibility.  This was not a compat break, because the old format was
never accepted by any version of libctf (the not-in-official-release CTF
compiler patch was emitting an invalid func info section), but
nonetheless it can confuse users using that patch if they link together
object files and find the func info sections in the inputs silently
disappearing.

Scan the linker inputs for this problem and emit a warning if any are
found.

libctf/ChangeLog
2021-01-05  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-link.c (ctf_link_warn_outdated_inputs): New.
	(ctf_link_write): Call it.