* dwarf2.c (struct dwarf2_debug): Add fields for handling
	alternate debug info source.
	(dwarf_debug_sections): Add entries for alternate .debug_str and
	.debug_info sections.
	(dwarf_debug_section_enum): Likewise.
	(read_alt_indirect_string): New function.  Handles a
	DW_FORM_GNU_strp_alt attribute.
	(read_alt_indirect_ref): New function.  Handles a
	DW_FORM_GNU_ref_alt attribute.
	(read_attribute_value): Process DW_FORM_GNU_ref_alt and
	DW_FORM_GNU_strp_alt.
	(find_abstract_instance_name): Handle DW_FORM_GNU_ref_alt
	attributes.
	(_bfd_dwarf2_cleanup_debug_info): Free alternate debug info
	sources.
	* opncls.c (GNU_DEBUGALTLINK): Define.
	(bfd_get_alt_debug_link_info): New function.
	(separate_alt_debug_file_exists): New function.
	(find_separate_debug_file): Add parameters for fetch and check
	functions.
	(bfd_follow_gnu_debugaltlink): New function.
	* bfd-in2.h: Regenerate.