Extract the reliability sensitive piece of code from libjpeg-turbo and get rid of cpufreq-ljt-stress-test

The NEON optimized libjpeg-turbo decoder is [sensitive to wrong DVFS configuration (insufficient voltage) on ARM Cortex-A8 processors](http://permalink.gmane.org/gmane.comp.hardware.netbook.arm.sunxi/9632).

The cpufreq-ljt-stress-test script was a quick and dirty solution, but eventually we need something that is more convenient to use.
