malloc: Call tcache destructor in arena_thread_freeres

It does not make sense to register separate cleanup functions for arena
and tcache since they're always going to be called together.  Call the
tcache cleanup function from within arena_thread_freeres since it at
least makes the order of those cleanups clear in the code.

Reviewed-by: Siddhesh Poyarekar <siddhesh@sourceware.org>