libstdc++: Replace unnecessary uses of built-ins in testsuite

I don't see why we should rely on __builtin_memset etc. in tests. We can
just include <cstring> and use the public API.

libstdc++-v3/ChangeLog:

	* testsuite/23_containers/deque/allocator/default_init.cc: Use
	std::memset instead of __builtin_memset.
	* testsuite/23_containers/forward_list/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/list/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/map/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/set/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/unordered_map/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/unordered_set/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/vector/allocator/default_init.cc:
	Likewise.
	* testsuite/23_containers/vector/bool/allocator/default_init.cc:
	Likewise.
	* testsuite/29_atomics/atomic/compare_exchange_padding.cc:
	Likewise.
	* testsuite/util/atomic/wait_notify_util.h: Likewise.