Detect MIPS CPU options

A commit to the Linux kernel on 24 October 2021 extended /proc/cpuinfo on MIPS to include information about CPU options.

The CPU options include nan2008 (I.e. whether the FPU is supporting the 2008 version of the IEEE 754 standard or the earlier version). User space software sometimes needs to know this, so that it can use a math library appropriate for the floating point hardware.

To facilitate this, cpu_features should check for nan2008 in /proc/cpuinfo and set a flag in the options structure.

(if I have time, I'll write this and submit a pull request).