CONFIG_HOTPLUG_CPU is not available for thedefaultarchitecture x86.
Result is shown for architecture s390

Support for hot-pluggable CPUs

modulename: hotplug.ko

and/or

hotplug-cpu.ko

configname: CONFIG_HOTPLUG_CPU

Linux Kernel Configuration
└─>Base setup
└─>Support for hot-pluggable CPUs
In linux kernel since version 2.6.12  
Say Y here to be able to turn CPUs off and on. CPUs
can be controlled through /sys/devices/system/cpu/cpu#.
Say N if you want to disable CPU hotplug.

source code:
selects
CONFIG_HOTPLUG

depends
CONFIG_SMP