Support non-round-robin scheduling

[RalfJung]

Miri now randomly yields the current thread every now and then, but currently it always yields to the next thread, i.e. it's a round-robin scheduler. It would probably be good to randomize this more and pick some arbitrary unblocked thread whenever we have yielded.

To still provide a fixed schedule for testcases, we could have a flag -Zmiri-fixed-schedule that disables both automatic yielding and that forces things to be round-robin again. All existing tests that use -Zmiri-preemption-rate=0 should then use this new flag instead.

[geetanshjuneja]

Can we randomly pick the same thread that yielded or does it always have to be a new thread?

[RalfJung]

Yeah we should include that in the possible options.