libstdc++ Enhance thread safety of debug mode iterators

libstdc++ Enhance thread safety of debug mode iterators

Avoids race condition when checking for an iterator to be singular or
to be comparable to another iterator.

	* src/c++/debug.cc
	(_Safe_sequence_base::_M_attach_single): Set attached iterator
	sequence pointer and version.
	(_Safe_sequence_base::_M_detach_single): Reset detached iterator.
	(_Safe_iterator_base::_M_attach): Remove attached iterator sequence
	pointer and version asignments.
	(_Safe_iterator_base::_M_attach_single): Likewise.
	(_Safe_iterator_base::_M_detach_single): Remove detached iterator
	reset.
	(_Safe_iterator_base::_M_singular): Use atomic load to access parent
	sequence.
	(_Safe_iterator_base::_M_can_compare): Likewise.
	(_Safe_iterator_base::_M_get_mutex): Likewise.
	(_Safe_local_iterator_base::_M_attach): Remove attached iterator container
	pointer and version assignments.
	(_Safe_local_iterator_base::_M_attach_single): Likewise.
	(_Safe_unordered_container_base::_M_attach_local_single):
	Set attached iterator container pointer and version.
	(_Safe_unordered_container_base::_M_detach_local_single): Reset detached
	iterator.