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

Enable iWMMXt support

modulename: iwmmxt.ko

configname: CONFIG_IWMMXT

Linux Kernel Configuration
└─>System Type
└─>Enable iWMMXt support
In linux kernel since version 2.6.21 (release Date: 2007-04-25)  
Enable support for iWMMXt context switching at run time if
running on a CPU that supports it.

source code:
arch/arm/kernel/

is depended by
CONFIG_MACH_CENTRO