Hash tables currently store an evacuated bit in the low bit
of the overflow pointer.  That's probably not sustainable in the
long term as GC wants correctly typed & aligned pointers.  It is
also a pain to move any of this code to Go in the current state.

This change moves the evacuated bit into the tophash entries.

Performance change is negligable.

R=golang-dev, bradfitz
CC=golang-dev
https://golang.org/cl/14412043