csky: Fixup futex SMP implementation

Arnd said:
I would guess that for csky, this is a mistake, as the architecture
is fairly new and should be able to implement it.

Guo reply:
The c610, c807, c810 don't support SMP, so futex_cmpxchg_enabled = 1
with asm-generic's implementation.
For c860, there is no HAVE_FUTEX_CMPXCHG and cmpxchg_inatomic/inuser
implementation, so futex_cmpxchg_enabled = 0.

Thx for point it out, we'll implement cmpxchg_inatomic/inuser for
C860 and still use asm-generic for non-smp CPUs.

LTP test:
futex_wait01 1 TPASS : futex_wait(): errno=ETIMEDOUT(110): Connection timed out
futex_wait01 2 TPASS : futex_wait(): errno=EAGAIN/EWOULDBLOCK(11): Resource temporarily unavailable
futex_wait01 3 TPASS : futex_wait(): errno=ETIMEDOUT(110): Connection timed out
futex_wait01 4 TPASS : futex_wait(): errno=EAGAIN/EWOULDBLOCK(11): Resource temporarily unavailable
futex_wait02 1 TPASS : futex_wait() woken up
futex_wait03 1 TPASS : futex_wait() woken up
futex_wait04 1 TPASS : futex_wait() returned -1: errno=EAGAIN/EWOULDBLOCK(11): Resource temporarily unavailable

Signed-off-by: Guo Ren <guoren@linux.alibaba.com>
Cc: Arnd Bergmann <arnd@kernel.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Paul E. McKenney <paulmck@kernel.org>
Link: https://lore.kernel.org/lkml/CAK8P3a3+WaQNyJ6Za2qfu6=0mBgU1hApnRXrdp1b1=P7wwyRUg@mail.gmail.com/