riscv: enable HAVE_FUTEX_CMPXCHG if !SMP

We use the asm-generic implementation, so it will always be there.

XXX: this works around the fact that the probe trips up an unresolved
pagefaul with split supervisor/user address spaces.

Signed-off-by: Christoph Hellwig <hch@lst.de>
Signed-off-by: Palmer Dabbelt <palmer@sifive.com>