CONFIG_X86_64 is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

CONFIG_X86_64 (not configurable)

modulename: sys_x86_64.ko

and/or

vdso-image-64.ko

and/or

hv_apic.ko

configname: CONFIG_X86_64

Linux Kernel Configuration
└─>X86_64
In linux kernel since version 2.6.12  

depends
CONFIG_64BIT