Improve thread switching performance

This improves by a few percentage points, but it'll definitely depend on the game.

This bubbles up `threadReadyQueue[priority]` being the next issue with thread switching.  I tried creating a vector (which was slower due to iteration), and then a sparse vector (with a priority field and binary search), but it wasn't really a clear win. (https://github.com/unknownbrackets/ppsspp/commits/readyqueue-perf)

Anyway, it's better already this way.  Threads often push_front() so making that faster matters.

-[Unknown]