libctf: eliminate dtd_u, part 3: functions

One more member vanishes from the dtd_u, leaving only the member for
struct/union/enum members.

There's not much to do here, since as of commit afd78bd6 we use
the same representation (type sizes, etc) in the dtu_argv as we will
use in the final vlen, with one exception: the vlen has alignment
padding, and the dtu_argv did not.  Simplify things by adding suitable
padding in both cases.

libctf/ChangeLog
2021-03-18  Nick Alcock  <nick.alcock@oracle.com>

	* ctf-impl.h (ctf_dtdef_t) <dtd_u.dtu_argv>: Remove.
	* ctf-create.c (ctf_dtd_delete): No longer free it.
	(ctf_add_function): Use the dtd_vlen, not dtu_argv.  Properly align.
	* ctf-serialize.c (ctf_emit_type_sect): Just copy the dtd_vlen.
	* ctf-types.c (ctf_func_type_info): Just use the vlen.
	(ctf_func_type_args): Likewise.