CONFIG_ARCH_SPACEMIT is not available for thedefaultarchitecture x86.
Result is shown for architecture riscv

SpacemiT SoCs

configname: CONFIG_ARCH_SPACEMIT

Linux Kernel Configuration
└─>SoC selection
└─>SpacemiT SoCs
In linux kernel since version 5.4.292 (release Date: 2025-04-10)  
This enables support for SpacemiT SoC platform hardware.