CONFIG_PPC64 is not available for thedefaultarchitecture x86.
Result is shown for architecture powerpc

64-bit kernel

modulename: setup_64.ko

and/or

vdso64_wrapper.ko

and/or

head_64.ko

and/or

dma-iommu.ko

and/or

entry_64.ko

configname: CONFIG_PPC64

Linux Kernel Configuration
└─>64-bit kernel
In linux kernel since version 2.6.15 (release Date: 2006-01-02)  
This option selects whether a 32-bit or a 64-bit kernel
will be built.

source code:
arch/powerpc/kernel/