fix cancellation failure in single-threaded programs

even a single-threaded program can be cancellable, e.g. if it's called
pthread_cancel(pthread_self()). the correct predicate to check is not
whether multiple threads have been invoked, but whether pthread_self
has been invoked.

src/thread/cancel_impl.c

@@ -17,7 +17,7 @@ long (__syscall_cp)(long nr, long u, long v, long w, long x, long y, long z)
 	uintptr_t old_sp, old_ip;
 	long r;
 
-	if (!libc.threaded || (self = __pthread_self())->canceldisable)
+	if (!libc.main_thread || (self = __pthread_self())->canceldisable)
 		return __syscall(nr, u, v, w, x, y, z);
 
 	old_sp = self->cp_sp;