Avoid unnecessary clock_gettime() syscalls in GC stats.
This looks fine to me, but I really sort of hate #if 0, because that's just asking for eventual bitrot (and confusion once that happens). Why not just eliminate gc_start_thread_cpu entirely and be done with it?
@simonmar you're the RTS master, so I'll defer to you on this one. I just hate having dead code laying to rot.