CONFIG_ARCH_IXP4XX is not available for the selected architecture x86.
Result is shown for architecture arm

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.