Support reading undefined symbols from an ELF file

Support reading undefined symbols from an ELF file

In preparation to support the upcoming 'abicompat' tool, we need the
ABI corpus type to capture the symbols that are undefined in a given
ELF file.

This patch changes the ELF reader to capture the undefined symbols as
well as the symbol versions they need to be satisfied.  These
undefined symbols are then stored in the instance ABI corpus build as
a result of the reading of the ELF file.

At the moment, these undefined symbols are not serialized to the
native XML format of libabigail yet.  I guess I'll do that in a
subsequent patch.

	* include/abg-corpus.h (corpus::{set_undefined_fun_symbol_map,
	set_undefined_var_symbol_map, get_undefined_fun_symbol_map_sptr,
	get_undefined_fun_symbol_map, get_sorted_undefined_fun_symbols,
	get_undefined_var_symbol_map_sptr, get_undefined_var_symbol_map,
	get_sorted_undefined_var_symbols}): Declare new methods ...
	* src/abg-corpus.cc (corpus::{set_undefined_fun_symbol_map,
	set_undefined_var_symbol_map, get_undefined_fun_symbol_map_sptr,
	get_undefined_fun_symbol_map, get_sorted_undefined_fun_symbols,
	get_undefined_var_symbol_map_sptr, get_undefined_var_symbol_map,
	get_sorted_undefined_var_symbols}): ... and define them.
	(struct corpus::priv::{undefined_var_symbol_map,
	sorted_undefined_var_symbols, undefined_fun_symbol_map,
	sorted_undefined_fun_symbols}): New data members.
	* src/abg-dwarf-reader.cc (get_symbol_versionning_sections): Also
	return the SHT_GNU_verneed section.
	(get_version_needed_for_versym): New static function.
	(get_version_definition_for_versym): Factorize this function out
	of ..
	(get_version_for_symbol): ... this one.  Take a flag that says if
	we want the definition version or the needed version of a symbol.
	Extend the implementation using the two new function
	get_version_needed_for_versym() and
	get_version_definition_for_versym() above.  This function now
	returns the version either for a defined & exported symbol, or for
	an undefined symbol.
	(lookup_symbol_from_sysv_hash_tab)
	(lookup_symbol_from_gnu_hash_tab, lookup_symbol_from_symtab):
	Adjust for the change of signature of get_version_for_symbol().
	(read_context::{undefined_fun_syms_, undefined_var_syms_}): New
	data members.
	(read_context::lookup_elf_symbol_from_index): Adjust for
	invocation of the new signature of get_version_for_symbol().
	(read_context::{undefined_fun_syms_sptr, undefined_fun_syms,
	undefined_var_syms_sptr, undefined_var_syms}): Define new methods.
	(read_context::load_symbol_maps): Add support for loading
	undefined symbols and their versions.
	(read_context::maybe_load_symbol_maps):  Take in account the need
	to load undefined symbols as well.
	(read_corpus_from_elf): Once the undefined symbols have been read
	from the ELF file, stuff them into the resulting ABI corpus that
	has been built.

Signed-off-by: Dodji Seketeli <dodji@redhat.com>