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: Piyush swami <Piyush.swami@arm.com>
Signed-off-by: Vladimir Murzin <vladimir.murzin@arm.com>
Signed-off-by: Will Deacon <will@kernel.org>