write compilation unit and cu index

Compilation units in DWARF objects with matching `DwoId`s should be
appended to the `.debug_info` section in the output object.
Contributions from that unit to all sections are then added to a
`.debug_cu_index` section, currently being generated in the GNU
extension V2 format (like the string offset table).

Signed-off-by: David Wood <david.wood@huawei.com>

Author: davidtwco