Add 'Thread_base::join()'

Using the new 'join()' function, the caller can explicitly block for the
completion of the thread's 'entry()' function. The test case for this
feature can be found at 'os/src/test/thread_join'. For hybrid
Linux/Genode programs, the 'Thread_base::join()' does not map directly
to 'pthread_join'. The latter function gets already called by the
destructor of 'Thread_base'. According to the documentation, subsequent
calls of 'pthread_join' for one thread may result in undefined behaviour.
So we use a 'Genode::Lock' on this platform, which is in line with the
other platforms.

Related to #194, #501

base-codezero/src/base/thread/thread_start.cc

@@ -30,6 +30,7 @@ void Thread_base::_thread_start()
 {
 	Thread_base::myself()->_thread_bootstrap();
 	Thread_base::myself()->entry();
+	Thread_base::myself()->_join_lock.unlock();
 	Genode::sleep_forever();
 }