eina_cpu: Add an internal api for getting a random fast core

eina_cpu: Add an internal api for getting a random fast core

In a big.LITTLE ARM system cores can have different capabilities.  This
gives an internal API that randomly returns the core id of any of the
system's fastest cores.

On systems where all cores are the same, it will return any available core.

If we don't have cpufreq support we just return 0