libdw: Make sure that every debug_types sig8 is hashed

libdw: Make sure that every debug_types sig8 is hashed

When dwarf_formref_die can't find a sig8 in the hash, it walks
__libdw_intern_next_unit, and was then adding those to the hash.
However, if dwarf_offdie_types is called earlier, which also uses that
next_unit, then they are missed from the hash (and never revisited).

This patch makes __libdw_intern_next_unit do the sig8 hash insert, so no
type unit is ever missed.

Signed-off-by: Josh Stone <jistone@redhat.com>