libstdc++: Add missing feature-test macro in various headers

version.syn#2 requires various headers to define
__cpp_lib_allocator_traits_is_always_equal. Currently, only <memory> was
defining this macro. Implement fixes for the other headers as well.

Signed-off-by: Dhruv Chawla <dhruvc@nvidia.com>

libstdc++-v3/ChangeLog:

	* include/std/deque: Define macro
	__glibcxx_want_allocator_traits_is_always_equal.
	* include/std/forward_list: Likewise.
	* include/std/list: Likewise.
	* include/std/map: Likewise.
	* include/std/scoped_allocator: Likewise.
	* include/std/set: Likewise.
	* include/std/string: Likewise.
	* include/std/unordered_map: Likewise.
	* include/std/unordered_set: Likewise.
	* include/std/vector: Likewise.
	* testsuite/20_util/headers/memory/version.cc: New test.
	* testsuite/20_util/scoped_allocator/version.cc: Likewise.
	* testsuite/21_strings/headers/string/version.cc: Likewise.
	* testsuite/23_containers/deque/version.cc: Likewise.
	* testsuite/23_containers/forward_list/version.cc: Likewise.
	* testsuite/23_containers/list/version.cc: Likewise.
	* testsuite/23_containers/map/version.cc: Likewise.
	* testsuite/23_containers/set/version.cc: Likewise.
	* testsuite/23_containers/unordered_map/version.cc: Likewise.
	* testsuite/23_containers/unordered_set/version.cc: Likewise.
	* testsuite/23_containers/vector/version.cc: Likewise.