CONFIG_ARCH_IXP4XX is not available for thedefaultarchitecture x86.
Result is shown for architecture arm
default or selected kernelversion does not have config value CONFIG_ARCH_IXP4XX.
Result is shown for kernelversion 6.9-rc5

IXP4xx-based platforms

configname: CONFIG_ARCH_IXP4XX

Linux Kernel Configuration
└─>IXP4xx-based platforms
In linux kernel since version 2.6.12  
Support for Intel's IXP4XX (XScale) family of processors.