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/LibPthread/pthread.cpp

@@ -83,6 +83,7 @@ static int create_thread(void* (*entry)(void*), void* argument, PthreadAttrImpl*
     return syscall(SC_create_thread, pthread_create_helper, thread_params);
 }
 
+[[noreturn]] 	
 static void exit_thread(void* code)
 {
     syscall(SC_exit_thread, code);