libstdc++: Fix failing tests for AIX

These two tests fail on AIX because <sys/thread.h> defines struct thread
in the global namespace (despite it not being a reserved name). That
means the using-declaration that adds it to the global namespace causes
a redeclaration error.

libstdc++-v3/ChangeLog:

	* testsuite/30_threads/thread/cons/84535.cc: Use a custom
	namespace.
	* testsuite/30_threads/thread/cons/lwg2097.cc: Likewise.