libstdc++: Add effective-target for std::allocator implementation

libstdc++: Add effective-target for std::allocator implementation

This allows tests to be skipped if the std::allocator implementation is
not __gnu_cxx::new_allocator.

The 20_util/allocator/overaligned.cc test requires either C++17 or
new_allocator, otherwise we can't guarantee to return overaligned
memory.

libstdc++-v3/ChangeLog:

	* testsuite/18_support/50594.cc: Check effective target.
	* testsuite/20_util/allocator/1.cc: Likewise.
	* testsuite/20_util/allocator/overaligned.cc: Likewise.
	* testsuite/23_containers/unordered_map/96088.cc: Likewise.
	* testsuite/23_containers/unordered_multimap/96088.cc: Likewise.
	* testsuite/23_containers/unordered_multiset/96088.cc: Likewise.
	* testsuite/23_containers/unordered_set/96088.cc: Likewise.
	* testsuite/ext/throw_allocator/check_delete.cc: Likewise.
	* testsuite/ext/throw_allocator/check_new.cc: Likewise.
	* testsuite/lib/libstdc++.exp (check_effective_target_std_allocator_new):
	Define new proc.