CONFIG_CPU_SUBTYPE_MXG is not available for thedefaultarchitecture x86.
Result is shown for architecture sh

Support MX-G processor

modulename: setup-mxg.ko

configname: CONFIG_CPU_SUBTYPE_MXG

Processor sub-type selection
└─>Support MX-G processor
In linux kernel since version 2.6.26 (release Date: 2008-07-13)  
Select MX-G if running on an R8A03022BG part.

source code:
arch/sh/kernel/cpu/sh2a/