misc/cgo/testcarchive: fix path of libgo.a for darwin/arm

After CL 22461, c-archive build on darwin/arm is by default compiled
with -shared, so update the install path.

Fix build.

Change-Id: Ie93dbd226ed416b834da0234210f4b98bc0e3606
Reviewed-on: https://go-review.googlesource.com/22507Reviewed-by: David Crawshaw <crawshaw@golang.org>
Run-TryBot: David Crawshaw <crawshaw@golang.org>
TryBot-Result: Gobot Gobot <gobot@golang.org>

misc/cgo/testcarchive/carchive_test.go

@@ -147,7 +147,11 @@ func TestInstall(t *testing.T) {
 		t.Fatal(err)
 	}
 
-	compilemain(t, filepath.Join("pkg", GOOS+"_"+GOARCH, "libgo.a"))
+	libgopath := filepath.Join("pkg", GOOS+"_"+GOARCH, "libgo.a")
+	if GOOS == "darwin" && GOARCH == "arm" {
+		libgopath = filepath.Join("pkg", GOOS+"_"+GOARCH+"_shared", "libgo.a")
+	}
+	compilemain(t, libgopath)
 
 	binArgs := append(bin, "arg1", "arg2")
 	if out, err := exec.Command(binArgs[0], binArgs[1:]...).CombinedOutput(); err != nil {