Fix readelf's and objdump's dislplay of DWO links when multiple links are present.

 PR 26829
 * dwarf.c (struct dwo_info): Add cu_offset field.
 (add_dwo_info): Add cu_offset parameter.  Record in new dwo_info
 struct.
 (add_dwo_name): Add cu_offset field.
 (add_dwo_dir): Add cu_offset field.
 (add_dwo_id): Add cu_offset field.
 (read_and_display_attr_value): Pass cu_offset to dwo recording
 functions.
 (load_separate_debug_files): Accumulate name, dir and id values
 and display once for each CU.
 * testsuite/binutils-all/dwo.sL Use a separate CU for the second
 dwo link.
 * testsuite/binutils-all/readelf.k2: Update expected output.