CONFIG_NIOS2_HW_MULX_SUPPORT is not available for thedefaultarchitecture x86.
Result is shown for architecture nios2

Enable MULX instruction

configname: CONFIG_NIOS2_HW_MULX_SUPPORT

Linux Kernel Configuration
└─>Platform options
└─>Enable MULX instruction
In linux kernel since version 3.19 (release Date: 2015-02-08)  
Set to true if you configured the Nios II to include the MULX
instruction. Enables the -mhw-mulx compiler flag.