Fix scheduling when starting a thread + potential crash

When starting a better-priority thread, the previous thread is put at the end of the queue, not at the start.  Found this while debugging Crisis Core with JpcspTrace (which is neat, but crashes things if not used carefully...)

Also fix mipsCalls.get() to not insert when not found.  Callers were expecting this behavior.  Caused a crash saving state for me.

-[Unknown]