CONFIG_NIOS2_BMX_SUPPORT is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

Enable BMX instructions


Linux Kernel Configuration
└─>Platform options
└─>Enable BMX 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 BMX Bit Manipulation Extension instructions. Enables
the -mbmx compiler flag.