cmd/go: change ar argument to rc

Put 'r' first because that is the command, and 'c' is the modifier.
Keep 'c' because it means to not warn when creating an archive.
Drop 'u' because it is unnecessary and fails on Arch Linux.

No test because this is only for gccgo (I tested it manually).

Fixes #12310.

Change-Id: Id740257fb1c347dfaa60f7d613af2897dae2c059
Reviewed-on: https://go-review.googlesource.com/16664
Run-TryBot: Ian Lance Taylor <iant@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: David Crawshaw <crawshaw@golang.org>

src/cmd/go/build.go

@@ -2506,7 +2506,7 @@ func (gccgoToolchain) pack(b *builder, p *Package, objDir, afile string, ofiles
 	for _, f := range ofiles {
 		absOfiles = append(absOfiles, mkAbs(objDir, f))
 	}
-	return b.run(p.Dir, p.ImportPath, nil, "ar", "cru", mkAbs(objDir, afile), absOfiles)
+	return b.run(p.Dir, p.ImportPath, nil, "ar", "rc", mkAbs(objDir, afile), absOfiles)
 }
 
 func (tools gccgoToolchain) ld(b *builder, root *action, out string, allactions []*action, mainpkg string, ofiles []string) error {