libstdc++: Remove unconditional use of atomics in Debug Mode

libstdc++: Remove unconditional use of atomics in Debug Mode

The fix for PR 91910 (r10-3426-gf7a3a382279585) introduced unconditional
uses of atomics into src/c++11/debug.cc, which causes linker errors for
arm4t where GCC emits an unresolved reference to __sync_synchronize.

By making the uses of atomics depend on _GLIBCXX_HAS_GTHREADS we can
avoid those unconditional references to __sync_synchronize for targets
where the atomics are unnecessary. As a minor performance optimization
we can also check the __gnu_cxx::__is_single_threaded function to avoid
atomics for single-threaded programs even where they don't cause linker
errors.

libstdc++-v3/ChangeLog:

	* src/c++11/debug.cc (acquire_sequence_ptr_for_lock): New
	function.
	(reset_sequence_ptr): New function.
	(_Safe_iterator_base::_M_detach)
	(_Safe_local_iterator_base::_M_detach): Replace bare atomic_load
	with acquire_sequence_ptr_for_lock.
	(_Safe_iterator_base::_M_reset): Replace bare atomic_store with
	reset_sequence_ptr.