CONFIG_AMD_IOMMU is not available for thedefaultarchitecture x86.
Result is shown for architecture 64BIT

AMD IOMMU support

modulename: iommu.ko

configname: CONFIG_AMD_IOMMU

Linux Kernel Configuration
└─>Device Drivers
└─>IOMMU Hardware Support
└─>PCI support
└─>AMD IOMMU support
In linux kernel since version 2.6.27 (release Date: 2008-10-09)  
With this option you can enable support for AMD IOMMU hardware in
your system. An IOMMU is a hardware component which provides
remapping of DMA memory accesses from devices. With an AMD IOMMU you
can isolate the DMA memory of different devices and protect the
system from misbehaving device drivers or hardware.

You can find out if your system has an AMD IOMMU if you look into
your BIOS for an option to enable it or if you have an IVRS ACPI

source code: