CONFIG_NIOS2_CDX_SUPPORT is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

Enable CDX instructions


Linux Kernel Configuration
└─>Platform options
└─>Enable CDX instructions
In linux kernel since version 4.2 (release Date: 2015-08-30)  
Set to true if you configured the Nios II R2 to include
the CDX Bit Manipulation Extension instructions. Enables
the -mcdx compiler flag.