libc/libpthread: Inject threadsafe locking callbacks for rtld.

libc/libpthread: Inject threadsafe locking callbacks for rtld.

* Make rtld thread-safe. Add lib/libthread_xu/thread/thr_rtld.c
  to the build and use it to inject pthread based locking callbacks
  for rtld.

* In rtld_lock.c, do the thread_mask_clear(lock->mask); after releasing
  the actual lock, to avoid deadlocks when rtld is called recursively
  from the pthread locking callbacks.
  XXX An alternative solution for this issue would be, to flag the
      libpthread dynamic library as bind_now to avoid the recursive rtld
      calls.