runtime: always incorporate hash seed at start of hash computation

Otherwise we can get predictable collisions.

R=golang-dev, dave, patrick, rsc
CC=golang-dev
https://codereview.appspot.com/7051043
cmd/dist: sse auto-detect

R=golang-dev, dsymonds, minux.ma, iant, alex.brainman
CC=golang-dev
https://codereview.appspot.com/7035055