xtensa: nommu: select HAVE_FUTEX_CMPXCHG

Not having HAVE_FUTEX_CMPXCHG makes futex_detect_cmpxchg probe
cmpxchg_futex_value_locked with NULL address. It's not guaranteed to
fault without MMU, instead it locks up on Xtensa when there's no RAM at
address 0.

Select HAVE_FUTEX_CMPXCHG in noMMU Xtensa configurations.

Signed-off-by: Max Filippov <jcmvbkbc@gmail.com>