runtime: record $GOROOT_FINAL for runtime.GOROOT
runtime: record $GOROOT_FINAL for runtime.GOROOT

Update issue 1527.

R=adg, oerdnj
CC=golang-dev
http://codereview.appspot.com/4171060
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go