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

Freescale i.MX family

configname: CONFIG_ARCH_MXC

Linux Kernel Configuration
└─>Freescale i.MX family
In linux kernel since version 2.6.23 (release Date: 2007-10-09)  
Support for Freescale MXC/iMX-based family of processors