Modified changes for split-dwarf and dwarf-5.
* dwarf.c(process_debug_info): Include DW_TAG_skeleton_unit.
(display_debug_str_offsets): While dumping .debug_str_offsets.dwo,
pass proper str_offsets_base to fetch_indexed_string().
(load_separate_debug_files): Skip DWO ID dump for dwarf-5.