Fix last leak detected by Valgrind

Fix last leak detected by Valgrind

This was subtle and google was my friend.

The leak was in _dl_allocate_tls called by pthread_create() and
is due to the fact that threads are created in joinable state so that
once terminated are not freed. To make the thread to release
its resources upon termination we should set them in detached state.

No functional change.

Signed-off-by: Marco Costalba <mcostalba@gmail.com>