runtime: don't store evacuate bit as low bit of hashtable overflow pointer.

runtime: don't store evacuate bit as low bit of hashtable overflow pointer.

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