Handle DW_FORM_GNU_ref_alt which references the .debug_info section in the
.gnu_debugaltlink file.

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

	PR libbacktrace/82857
	* dwarf.c (enum attr_val_encoding): Add ATTR_VAL_REF_ALT_INFO.
	(read_attribute): Handle DW_FORM_GNU_ref_alt using
	ATTR_VAL_REF_ALT_INFO.
	(read_referenced_name_from_attr): Handle DW_FORM_GNU_ref_alt.

From-SVN: r268031