CONFIG_CPU_MIPS64 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

CONFIG_CPU_MIPS64 (not configurable)

modulename: r4k_fpu.ko

and/or

c-r4k.ko

and/or

dump_tlb.ko

configname: CONFIG_CPU_MIPS64

Linux/MIPS Kernel Configuration
└─>CPU_MIPS64
In linux kernel since version 2.6.12  
source code:
arch/mips/mm/