cmd/go: Use exported CgoLDFlags when compiler=gccgo

If you compile a program that has cgo LDFLAGS directives, those are exported to an environment variable to be used by subsequent compiler tool invocations. The linking phase when using the gccgo toolchain did not consider the envvar CGO_LDFLAGS's linking directives resulting in undefined references when using cgo+gccgo.

Fixes #7573

LGTM=iant
R=golang-codereviews, iant
CC=golang-codereviews
https://golang.org/cl/80780043

src/cmd/go/build.go

@@ -1905,6 +1905,7 @@ func (tools gccgoToolchain) ld(b *builder, p *Package, out string, allactions []
 	ldflags = append(ldflags, afiles...)
 	ldflags = append(ldflags, sfiles...)
 	ldflags = append(ldflags, cgoldflags...)
+	ldflags = append(ldflags, p.CgoLDFLAGS...)
 	if usesCgo && goos == "linux" {
 		ldflags = append(ldflags, "-Wl,-E")
 	}

src/cmd/go/test.bash

@@ -669,6 +669,31 @@ if ! ./testgo test -c -test.bench=XXX fmt; then
 fi
 rm -f fmt.test
 
+TEST 'Issue 7573: cmd/cgo: undefined reference when linking a C-library using gccgo'
+d=$(mktemp -d -t testgoXXX)
+export GOPATH=$d
+mkdir -p $d/src/cgoref
+ldflags="-L alibpath -lalib"
+echo "
+package main
+// #cgo LDFLAGS: $ldflags
+// void f(void) {}
+import \"C\"
+
+func main() { C.f() }
+" >$d/src/cgoref/cgoref.go
+go_cmds="$(./testgo build -n -compiler gccgo cgoref 2>&1 1>/dev/null)"
+ldflags_count="$(echo "$go_cmds" | egrep -c "^gccgo.*$(echo $ldflags | sed -e 's/-/\\-/g')" || true)"
+if [ "$ldflags_count" -lt 1 ]; then
+	echo "No Go-inline "#cgo LDFLAGS:" (\"$ldflags\") passed to gccgo linking stage."
+	ok=false
+fi
+rm -rf $d
+unset ldflags_count
+unset go_cmds
+unset ldflags
+unset GOPATH
+
 # clean up
 if $started; then stop; fi
 rm -rf testdata/bin testdata/bin1