cmd/go: silence ar with D flag failures

The first call of ar must not show its output in order to avoid useless
error messages about D flag.

Change-Id: I3a2f5144b3bb271705000b67cd46cd02e98aca77
Reviewed-on: https://go-review.googlesource.com/c/go/+/182077
Run-TryBot: Clément Chigot <clement.chigot@atos.net>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/cmd/go/internal/work/gccgo.go

@@ -207,9 +207,16 @@ func (tools gccgoToolchain) pack(b *Builder, a *Action, afile string, ofiles []s
 	}
 	absAfile := mkAbs(objdir, afile)
 	// Try with D modifier first, then without if that fails.
-	if b.run(a, p.Dir, p.ImportPath, nil, tools.ar(), arArgs, "rcD", absAfile, absOfiles) != nil {
+	output, err := b.runOut(a, p.Dir, nil, tools.ar(), arArgs, "rcD", absAfile, absOfiles)
+	if err != nil {
 		return b.run(a, p.Dir, p.ImportPath, nil, tools.ar(), arArgs, "rc", absAfile, absOfiles)
 	}
+
+	if len(output) > 0 {
+		// Show the output if there is any even without errors.
+		b.showOutput(a, p.Dir, p.ImportPath, b.processOutput(output))
+	}
+
 	return nil
 }