Handle DW_FORM_GNU_strp_alt which references the .debug_str section in the
.gnu_debugaltlink file.

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

	PR libbacktrace/82857
	* dwarf.c (read_attribute): Handle DW_FORM_GNU_strp_alt
	using altlink.

From-SVN: r267996