[libbacktrace] Add find_unit

[libbacktrace] Add find_unit

Add a function that finds the unit given an offset into .debug_info.

2019-01-17  Tom de Vries  <tdevries@suse.de>

	* dwarf.c (struct unit): Add low_offset and high_offset fields.
	(struct unit_vector): New type.
	(struct dwarf_data): Add units and units_counts fields.
	(find_unit): New function.
	(find_address_ranges): Add and handle unit_tag parameter.
	(build_address_map): Add and handle units_vec parameter.
	(build_dwarf_data): Pass units_vec to build_address_map.  Store resulting
	units vector.

From-SVN: r268030