libctf, ld: fix formatting of forwards to unions and enums

The type printer was unconditionally printing these as if they were
forwards to structs, even if they were forwards to unions or enums.

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

	* testsuite/ld-ctf/enum-forward.c: New test.
	* testsuite/ld-ctf/enum-forward.c: New results.

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

	* ctf-types.c (ctf_type_aname): Print forwards to unions and enums
	properly.