nptl: Change type of __default_pthread_attr

union pthread_attr_transparent has always the correct size, even if
pthread_attr_t has padding that is not present in struct pthread_attr.

This should not result in an observable behavioral change.  The
existing code appears to have been correct, but it was brittle because
it was not clear which functions were allowed to write to an entire
pthread_attr_t argument (e.g., by copying it).

Reviewed-by: Carlos O'Donell <carlos@redhat.com>