Get cpu core count info from /sys where possible

Fixes #4215.  May impact texture scaling performance.

While testing this, I realized that my Android device, even with multithreading enabled (io and cpu) and unthrottled, only has one core actually online, according to /sys/devices/system/cpu/online.  If I'm correct about that, it's probably why multithreading doesn't help Android at all currently.

One link online said that it doesn't use the extra core if you start a thread while the current core is not maxed out.  All the threads are started at startup so...

-[Unknown]