CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG (not configurable)

configname: CONFIG_NEEDS_SYSCALL_FOR_CMPXCHG

Linux Kernel Configuration
└─>NEEDS_SYSCALL_FOR_CMPXCHG
In linux kernel since version 2.6.12  
SMP on a pre-ARMv6 processor? Well OK then.
Forget about fast user space cmpxchg support.
It is just not possible.