arm64: Kconfig: select HAVE_FUTEX_CMPXCHG

arm64 provides always working implementation of futex_atomic_cmpxchg_inatomic(),
so there is no need to check it runtime.
Reported-by: NPiyush swami <Piyush.swami@arm.com>
Signed-off-by: NVladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: NWill Deacon <will@kernel.org>

arch/arm64/Kconfig

@@ -161,6 +161,7 @@ config ARM64
 	select HAVE_PERF_USER_STACK_DUMP
 	select HAVE_REGS_AND_STACK_ACCESS_API
 	select HAVE_FUNCTION_ARG_ACCESS_API
+	select HAVE_FUTEX_CMPXCHG if FUTEX
 	select HAVE_RCU_TABLE_FREE
 	select HAVE_RSEQ
 	select HAVE_STACKPROTECTOR