libdw: Recognize GNU DebugFission split units.

The split dwarf dwo unit id and type are not in the CU header itself, but
can be found in the CU DIE DW_AT_GNU_dwo attributes. Use this to set the
correct unit_type and id for GNU DebugFission split units. Also show this
information in eu-readelf when printing units.

Signed-off-by: Mark Wielaard <mark@klomp.org>
5 files changed