CONFIG_CPU_S3C6400 is not available for thedefaultarchitecture x86.
Result is shown for architecture arm

CONFIG_CPU_S3C6400 (not configurable)

modulename: s3c6400.ko

and/or

configname: CONFIG_CPU_S3C6400

Linux Kernel Configuration
└─>CPU_S3C6400
In linux kernel since version 2.6.31 (release Date: 2009-09-09)  
Enable S3C6400 CPU support

source code:
arch/arm/mach-s3c64xx/s3c6400.c