Add handling of the DW_FORM_ref_addr encoding to libbacktrace.

2019-02-08  Tom de Vries  <tdevries@suse.de>

	PR libbacktrace/78063
	* dwarf.c (build_address_map): Keep all parsed units.
	(read_referenced_name_from_attr): Handle DW_FORM_ref_addr.

From-SVN: r268663