build: add GO_ prefix to LDFLAGS and GCFLAGS
build: add GO_ prefix to LDFLAGS and GCFLAGS

Build environments will often define stock LDFLAGS
that are not compatible with the gc ld, causing
non-obvious failures midway through the build.

R=golang-dev, rsc, minux.ma, rsc
CC=golang-dev
http://codereview.appspot.com/5724044
abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go,abustany/go