libstdc++: Refactor std::list::size() for cxx11 ABI

libstdc++: Refactor std::list::size() for cxx11 ABI

Remove some preprocessor conditionals by moving the _M_size member for
the cxx11 ABI into a new base class, which is empty for the gcc4-compat
ABI.

Move some unused members that are only retained for ABI compatibility to
the end of _List_base and add an explanatory comment. Stop using
list::_M_node_count and list::_D_distance and then move them to the end
of std::list with a comment too.

libstdc++-v3/ChangeLog:

	* include/bits/stl_list.h (_List_size): New struct.
	(_List_node_header): Replace _M_size member with _List_size base
	class.
	(_List_node_header(_List_node_header&&)): Replace explicit uses
	of _M_size with initializing the base.
	(_List_node_header::_M_init): Likewise.
	(_List_base::_S_distance, _List_base::_M_distance)
	(_List_base::_M_node_count): Move to end of class body and add
	comment.
	(list::_S_distance, list::_M_node_count): Likewise.
	(list::size): Inline _M_node_count effects to here.
	(list::splice(iterator, list&, iterator, iterator)): Use #if and
	call std::distance instead of _S_distance.