[libstdc++] do not destruct mutex_pool mutexes

[libstdc++] do not destruct mutex_pool mutexes

On vxworks, after destroying the semaphore used to implement a mutex,
__gthread_mutex_lock fails and __gnu_cxx::__mutex::lock calls
__throw_concurrence_lock_error.  Nothing ensures the mutex_pool
mutexes survive init-once objects containing _Safe_sequence_base.  If
such an object completes construction before mutex_pool
initialization, it will be registered for atexit destruction after the
mutex_pool mutexes, so the _M_detach_all() call in the
_Safe_sequence_base dtor will use already-destructed mutexes, and
basic_string/requirements/citerators_cc fails calling terminate.

This patch fixes this problem by ensuring the mutex pool mutexes are
constructed on demand, on a statically-allocated buffer, but never
destructed.

for  libstdc++-v3/ChangeLog

	* src/c++11/shared_ptr.cc (__gnu_internal::get_mutex):
	Avoid destruction of the mutex pool.