objdump: get CTF parent importing right

The linker emits CTF into a single section named .ctf, which is a CTF
archive where the default member (itself named ".ctf", or simply NULL)
is the parent of all other members.  Teach objdump to look for this by
default, rather than only trying to do it if a specific CTF parent
section was specified.  (If no parent name is specified, we get the .ctf
member from the same section as everything else, which matches what the
linker generates.)

binutils/
	* objdump.c (dump_ctf): Use the default CTF archive member as the
	parent even when no parent section is specified.
	(dump_ctf_archive_member): Only import from the parent
	if this is not the default ".ctf" member.