Implement std::pmr::synchronized_pool_resource

Implement std::pmr::synchronized_pool_resource

Define the thread-safe pool resource, using a shared_mutex to allow
multiple threads to concurrently allocate from thread-specific pools.

Define new weak symbols for the pthread_rwlock_t functions, to avoid
making libstdc++.so depend on libpthread.so

When the necessary Gthread support is absent only define the
feature-test macro to 1, rather than 201603. This is intended to imply
incomplete support, because everything except synchronized_pool_resource
works.

	Implement std::pmr::synchronized_pool_resource
	* config/abi/pre/gnu.ver: Add new symbols.
	* include/std/memory_resource [_GLIBCXX_HAS_GTHREADS]
	(__cpp_lib_memory_resource): Define to expected value, 201603.
	(synchronized_pool_resource): New class.
	[!_GLIBCXX_HAS_GTHREADS] (__cpp_lib_memory_resource): Define to 1.
	* include/std/shared_mutex (__glibcxx_rwlock_rdlock)
	(__glibcxx_rwlock_tryrdlock, __glibcxx_rwlock_wrlock)
	(__glibcxx_rwlock_trywrlock, __glibcxx_rwlock_unlock)
	(__glibcxx_rwlock_destroy, __glibcxx_rwlock_init)
	(__glibcxx_rwlock_timedrdlock, __glibcxx_rwlock_timedwrlock): Define
	weak symbols for POSIX rwlock functions.
	(__shared_mutex_pthread): Use weak symbols.
	* include/std/version (__cpp_lib_memory_resource): Define.
	* src/c++17/memory_resource.cc [_GLIBCXX_HAS_GTHREADS]
	(synchronized_pool_resource::_TPools): New class.
	(destroy_TPools): New function for pthread_key_create destructor.
	(synchronized_pool_resource::synchronized_pool_resource)
	(synchronized_pool_resource::~synchronized_pool_resource)
	(synchronized_pool_resource::release)
	(synchronized_pool_resource::do_allocate)
	(synchronized_pool_resource::do_deallocate): Define public members.
	(synchronized_pool_resource::_M_thread_specific_pools)
	(synchronized_pool_resource::_M_alloc_tpools)
	(synchronized_pool_resource::_M_alloc_shared_tpools): Define private
	members.
	* testsuite/20_util/synchronized_pool_resource/allocate.cc: New test.
	* testsuite/20_util/synchronized_pool_resource/cons.cc: New test.
	* testsuite/20_util/synchronized_pool_resource/is_equal.cc: New test.
	* testsuite/20_util/synchronized_pool_resource/multithreaded.cc: New
	test.
	* testsuite/20_util/synchronized_pool_resource/release.cc: New test.
	* testsuite/performance/20_util/memory_resource/pools.cc: Add
	multithreaded tests using pmr::synchronized_pool_resource.

From-SVN: r266242