cmd/go: fix freebsd build

R=golang-dev, adg
CC=golang-dev
https://golang.org/cl/5530063

src/cmd/go/build.go

@@ -449,6 +449,9 @@ func (b *builder) do(root *action) {
 	// drop the parallelism to 1, both to make the output
 	// deterministic and because there is no real work anyway.
 	par := buildP
+	if par == 0 {
+		par = 1
+	}
 	if buildN {
 		par = 1
 	}