runtime: fix bug in maps at the intersection of iterators, growing, and NaN keys

If an iterator is started while a map is in the middle of a grow,
and the map has NaN keys, then those keys might get returned by
the iterator more than once.  This fix makes the evacuation decision
deterministic and repeatable for NaN keys so each one gets returned
only once.

R=golang-dev, r, khr, iant
CC=golang-dev
https://codereview.appspot.com/14367043