libstdc++: Fix some AIX test failures

libstdc++: Fix some AIX test failures

AIX <sys/thread.h> defines struct tstate with non-reserved names, so
adjust the 17_intro/names.cc test. It also defines struct user, which
conflicts with namespace user in some tests.

Replacing the global operator new doesn't work on AIX the same way as it
does for ELF, so skip some tests that depend on replacing it.

Add missing DG directives to synchronized_value test so it doesn't run
for the single-threaded AIX multilib.

libstdc++-v3/ChangeLog:

	* testsuite/17_intro/names.cc [_AIX]: Do not define policy.
	* testsuite/19_diagnostics/error_code/cons/lwg3629.cc: Rename
	namespace to avoid clashing with libc struct.
	* testsuite/19_diagnostics/error_condition/cons/lwg3629.cc:
	Likewise.
	* testsuite/23_containers/unordered_map/96088.cc: Skip on AIX.
	* 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/experimental/synchronized_value.cc: Require gthreads
	and add missing option for pthreads targets.