libbacktrace: remove duplicate low_pc/high_pc/range handling

libbacktrace: remove duplicate low_pc/high_pc/range handling

	* dwarf.c (struct pcrange): Define.
	(update_pcrange, add_ranges): New static functions.
	(add_unit_addr): Change signature to work with add_ranges.  Don't
	add base_address here.
	(add_unit_ranges): Remove.
	(find_address_ranges): Replace str/ranges parameters with
	dwarf_sections.  Use update_pcrange and add_ranges.  Change all
	callers.
	(add_function_range): Change signature to work with add_ranges.
	Don't add base_address here.
	(add_function_ranges): Remove.
	(read_function_entry): Use update_pcrange and add_ranges.

From-SVN: r279154