libdw: Add new function dwarf_cu_die.

libdw: Add new function dwarf_cu_die.

Given a Dwarf_Die or Dwarf_Attribute it is often convenient to get at the
CU DIE and the CU header information. There is dwarf_diecu but that doesn't
provide all information from the header and it doesn't work for attributes.
Add a new dwarf_cu_die function that provides all information given a
Dwarf_CU, which both Dwarf_Die and Dwarf_Attribute reference.

Signed-off-by: Mark Wielaard <mjw@redhat.com>