libdw: Search skeleton DIE for split compile unit DIE attributes.

libdw: Search skeleton DIE for split compile unit DIE attributes.

dwarf_attr_integrate and dwarf_hasattr_integrate should also search
for attributes from the skeleton DIE in case the given DIE is a split
compile unit DIE. Split compile unit DIEs inherit various attributes
from their skeleton unit DIE in DWARF5.

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