[libbacktrace] Handle alt FORMS without .gnu_debugaltlink

[libbacktrace] Handle alt FORMS without .gnu_debugaltlink

Handle DW_FORM_GNU_strp_alt and DW_FORM_GNU_ref_alt references robustly in
presence of missing .gnu_debugaltlink file.

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

	* dwarf.c (enum attr_val_encoding): Add ATTR_VAL_NONE.
	(read_attribute): Add altlink parameter.  Handle missing altlink for
	DW_FORM_GNU_strp_alt and DW_FORM_GNU_ref_alt.
	(find_address_ranges, build_address_map, build_dwarf_data): Add and
	handle altlink parameter.
	(read_referenced_name, read_function_entry): Add argument to
	read_attribute call.

From-SVN: r267995