make pthread_exit responsible for disabling cancellation

this requirement is tucked away in XSH 2.9.5 Thread Cancellation under
the heading Thread Cancellation Cleanup Handlers.

src/thread/pthread_cancel.c

@@ -3,9 +3,6 @@
 
 void __cancel()
 {
-	pthread_t self = __pthread_self();
-	self->canceldisable = 1;
-	self->cancelasync = 0;
 	pthread_exit(PTHREAD_CANCELED);
 }
 

src/thread/pthread_create.c

@@ -23,6 +23,8 @@ _Noreturn void __pthread_exit(void *result)
 	pthread_t self = __pthread_self();
 	sigset_t set;
 
+	self->canceldisable = 1;
+	self->cancelasync = 0;
 	self->result = result;
 
 	while (self->cancelbuf) {