readelf: Handle .debug_str_offsets.

readelf: Handle .debug_str_offsets.

The .debug_str_offsets tables are indirect string offsets into the
.debug_str section. For DWARF5 they can be in both the main, skeleton
and split dwarf (.dwo) files.

For DWARF4 with the GNU DebugFission extension the tables will not have
an header and they will only be in the split DWARF (.dwo) file, never in
the main (skeleton) file.

For DWARF5 the (non-split) unit DIE will have a DW_AT_str_offsets_base
attribute pointing at the actual index (after the header). The split
unit will never have this attribute (and use the table at offset zero).

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