build: update comment about GO_LDFLAGS. (5l/6l/8l aren't involved in building packages)
build: update comment about GO_LDFLAGS.
(5l/6l/8l aren't involved in building packages)

R=golang-dev, robert.hencke
CC=golang-dev
http://codereview.appspot.com/5798071
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go