DWARF5 defines a small header for .debug_str_offsets.  Since we only use
it for split dwarf .dwo files we don't need to keep track of the actual
index offset in an attribute.

gcc/ChangeLog

	* dwarf2out.c (count_index_strings): New function.
	(output_indirect_strings): Call count_index_strings and generate
	header for dwarf_version >= 5.

From-SVN: r260298