CONFIG_ISS_SMP_EXTN is not available for thedefaultarchitecture x86.
Result is shown for architecture arc
default or selected kernelversion does not have config value CONFIG_ISS_SMP_EXTN.
Result is shown for kernelversion 6.5.1

ARC SMP Extensions (ISS Models only)

configname: CONFIG_ISS_SMP_EXTN

Linux Kernel Configuration
└─>"Legacy" ARC FPGA dev Boards
└─>ARC SMP Extensions (ISS Models only)
In linux kernel since version 3.1 (release Date: 2011-10-24)  
SMP Extensions to ARC700, in a "simulation only" Model, supported in
ARC ISS (Instruction Set Simulator).
The SMP extensions include:
-IDU (Interrupt Distribution Unit)
-XTL (To enable CPU start/stop/set-PC for another CPU)
It doesn't provide coherent Caches and/or Atomic Ops (LLOCK/SCOND)