tst-clone3: Use __NR_futex_time64 if we don't have __NR_futex

We can't include sysdep.h in the test case (it introduces lots of
strange failures) so __NR_futex isn't redifined to __NR_futex_time64 by
64-bit time_t 32-bit archs (y2038 safe).

To allow the test to pass let's just do the __NR_futex_time64 syscall if
we don't have __NR_futex defined.

Reviewed-by: Adhemerval Zanella <adhemerval.zanella@linaro.org>