libstdc++: Do not use volatile for __gnu_cxx::rope reference counting

libstdc++: Do not use volatile for __gnu_cxx::rope reference counting

The rope extension uses a volatile variable for its reference count.
This is not only unnecessary for correctness (volatile provides neither
atomicity nor memory visibility, and the variable is only modified while
a lock is held) but it now causes deprecated warnings with
-Wsystem-headers due to the use of ++ and -- operators.

It would be possible to use __gnu_cxx::__exchange_and_add in _M_incr and
_M_decr when __atomic_is_lock_free(sizeof(_RC_t), &_M_ref_count) is
true, rather than locking a mutex. That would probably be a significant
improvement for multi-threaded and single-threaded code (because
__exchange_and_add will use non-atomic ops when possible, and even in MT
code it should be faster than the mutex lock/unlock pair). However,
mixing objects compiled with the old and new code would result in
inconsistent synchronization being used for the reference count.

libstdc++-v3/ChangeLog:

	* include/ext/rope (_Refcount_Base::_M_ref_count): Remove
	volatile qualifier.
	(_Refcount_Base::_M_decr()): Likewise.