CONFIG_ISS_SMP_EXTN is not available for the default architecture x86.
Result is shown for architecture arc

ARC SMP Extensions (ISS Models only)


Linux Kernel Configuration
└─> "Legacy" ARC FPGA dev Boards
└─> ARC SMP Extensions (ISS Models only)

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)