CONFIG_ARCH_MXC is not available for thedefaultarchitecture x86.
Result is shown for architecture arm64

ARMv8 based NXP i.MX SoC family

modulename: soc-imx8.ko

configname: CONFIG_ARCH_MXC

Linux Kernel Configuration
└─>Platform selection
└─>ARMv8 based NXP i.MX SoC family
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
This enables support for the ARMv8 based SoCs in the
NXP i.MX family.