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/include/base/thread.h

@@ -254,6 +254,11 @@ namespace Genode {
 			 */
 			Native_thread _tid;
 
+			/**
+			 * Lock used for synchronizing the finalization of the thread
+			 */
+			Genode::Lock _join_lock;
+
 		public:
 
 			/**
@@ -335,6 +340,14 @@ namespace Genode {
 			 * 0 when called by the main thread.
 			 */
 			Native_utcb *utcb();
+
+			/**
+			 * Block until the thread leaves the 'entry' function
+			 *
+			 * Join must not be called more than once. Subsequent calls have
+			 * undefined behaviour.
+			 */
+			void join();
 	};