[ARM PATCH] 1866/4: kernel support for iWMMXt present on some XScale cores

[ARM PATCH] 1866/4: kernel support for iWMMXt present on some XScale cores

Patch from Nicolas Pitre

This is required for a system with user space binaries using iWMMXt
instructions to even boot (revised again).

It also moves defines for thread_info offsets from  magic static
values to the auto generated asm/constants.h in order to better
cope with changing structure offsets and avoid user errors.