libdw: Add new dwarf_get_units function to iterate over all units.

libdw: Add new dwarf_get_units function to iterate over all units.

The dwarf_nextcu and dwarf_next_unit functions provide information to
construct the offset to construct the associated CU DIE using dwarf_offdie
or dwarf_offdie_types.  This requires the user to know beforehand where
to DIE data is stored (in the .debug_info or .debug_types section).
For type units one also needs to use the type offset to create the actual
type DIE. In DWARF5 DIEs can come from even more data locations. And there
are also skeleton units which require the user to find the associated
split compile unit DIE (which would come from a different file).

The new dwarf_get_units function simplifies iterating over the units in
a DWARF file. It doesn't require the user to know where the DIE data is
stored, it will automagically iterate over all know data sources (sections)
returning the Dwarf_CU and the associated Dwarf_Die if requested. If the
user requests to know the associated "subdie" it will also be resolved.

This implementation returns the correct subdie for type units. A future
version will also handle skeleton units and return the associated skeleton
DIE and split unit DIE.

readelf has been adapted to use the new iterator and print the new DWARF5
unit header information (which it gets through dwarf_cu_die). The new
interface hides which section exactly to iterate on (by design). readelf
works around that by "cheating". It sets up a Dwarf_CU so that it gets
the data from the right section, using the (normally) internal data
structure.

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