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>

arch/xtensa/Kconfig

@@ -17,6 +17,7 @@ config XTENSA
 	select HAVE_DMA_API_DEBUG
 	select HAVE_DMA_ATTRS
 	select HAVE_FUNCTION_TRACER
+	select HAVE_FUTEX_CMPXCHG if !MMU
 	select HAVE_IRQ_TIME_ACCOUNTING
 	select HAVE_OPROFILE
 	select HAVE_PERF_EVENTS