cmd/ld, runtime: restrict stack root scan to locals and arguments

Updates issue 5134

R=golang-dev, bradfitz, cshapiro, daniel.morsing, ality, iant
CC=golang-dev
https://codereview.appspot.com/8022044
cmd/godoc: provide a link from notes to source location

R=golang-dev, r
CC=golang-dev
https://codereview.appspot.com/8122043