LibPThread: Make pthread_exit a noreturn function

LibPThread: mark pthread_exit a noreturn function using compiler attributes
LibThread: remove a call to pthread_exit from Thread::start lambda expression
as it make the return of teh lambda unreachable.

Libraries/LibThread/Thread.cpp

@@ -51,8 +51,7 @@ void LibThread::Thread::start()
         [](void* arg) -> void* {
             Thread* self = static_cast<Thread*>(arg);
             size_t exit_code = self->m_action();
-            self->m_tid = 0;
-            pthread_exit((void*)exit_code);
+            self->m_tid = 0;            
             return (void*)exit_code;
         },
         static_cast<void*>(this));