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.

libstdc++-v3/include/ext/rope

@@ -452,7 +452,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     typedef std::size_t _RC_t;
     
     // The data member _M_ref_count
-    volatile _RC_t _M_ref_count;
+    _RC_t _M_ref_count;
 
     // Constructor
 #ifdef __GTHREAD_MUTEX_INIT
@@ -489,7 +489,7 @@ _GLIBCXX_BEGIN_NAMESPACE_VERSION
     _M_decr()
     {
       __gthread_mutex_lock(&_M_ref_count_lock);
-      volatile _RC_t __tmp = --_M_ref_count;
+      _RC_t __tmp = --_M_ref_count;
       __gthread_mutex_unlock(&_M_ref_count_lock);
       return __tmp;
     }