CONFIG_CPU_R8000 is not available for thedefaultarchitecture x86.
Result is shown for architecture mips

R8000

modulename: c-r4k.ko

and/or

r4k_fpu.ko

configname: CONFIG_CPU_R8000

CPU type
└─>R8000
In linux kernel since version 2.6.12  
MIPS Technologies R8000 processors. Note these processors are
uncommon and the support for them is incomplete.

source code:
arch/mips/mm/

depends
CONFIG_EXPERIMENTAL
CONFIG_SYS_HAS_CPU_R8000