cmd/go: print CC environment variables on Plan 9

This changes makes the output of `go env` the same
as on other operating systems.

Fixes #18013.

Change-Id: I3079e14dcf7b30c75ec3fde6c78cb95721111320
Reviewed-on: https://go-review.googlesource.com/33396Reviewed-by: Ian Lance Taylor <iant@golang.org>
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

src/cmd/go/env.go

@@ -62,13 +62,11 @@ func mkEnv() []envVar {
 		env = append(env, envVar{"GO386", os.Getenv("GO386")})
 	}
 
-	if goos != "plan9" {
-		cmd := b.gccCmd(".")
-		env = append(env, envVar{"CC", cmd[0]})
-		env = append(env, envVar{"GOGCCFLAGS", strings.Join(cmd[3:], " ")})
-		cmd = b.gxxCmd(".")
-		env = append(env, envVar{"CXX", cmd[0]})
-	}
+	cmd := b.gccCmd(".")
+	env = append(env, envVar{"CC", cmd[0]})
+	env = append(env, envVar{"GOGCCFLAGS", strings.Join(cmd[3:], " ")})
+	cmd = b.gxxCmd(".")
+	env = append(env, envVar{"CXX", cmd[0]})
 
 	if buildContext.CgoEnabled {
 		env = append(env, envVar{"CGO_ENABLED", "1"})