CONFIG_CPU_SUPPORTS_64BIT_KERNEL is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_CPU_SUPPORTS_64BIT_KERNEL (not configurable)

configname: CONFIG_CPU_SUPPORTS_64BIT_KERNEL

Linux Kernel Configuration
└─>CPU_SUPPORTS_64BIT_KERNEL
In linux kernel since version 2.6.14 (release Date: 2005-10-27)