fix failure of internal futex __timedwait to report ECANCELED

as part of abstracting the futex wait, this function suppresses all
futex error values which callers should not see using a whitelist
approach. when the masked cancellation mode was added, the new
ECANCELED error was not whitelisted. this omission caused the new
pthread_cond_wait code using masked cancellation to exhibit a spurious
wake (rather than acting on cancellation) when the request arrived
after blocking on the cond var.

src/thread/__timedwait.c

@@ -33,7 +33,7 @@ int __timedwait(volatile int *addr, int val,
 
 	r = -__syscall_cp(SYS_futex, addr, FUTEX_WAIT|priv, val, top);
 	if (r == ENOSYS) r = -__syscall_cp(SYS_futex, addr, FUTEX_WAIT, val, top);
-	if (r != EINTR && r != ETIMEDOUT) r = 0;
+	if (r != EINTR && r != ETIMEDOUT && r != ECANCELED) r = 0;
 
 	pthread_cleanup_pop(0);
 	if (!cleanup) __pthread_setcancelstate(cs, 0);