tstest/archtest: add GOARCH-specific tests, run via qemu-user

Updates #3233

Change-Id: Ia224c90490d41e50a1d547eeea709b0d9171c1f9
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

tstest/archtest/archtest_test.go

@@ -0,0 +1,32 @@
+// Copyright (c) 2021 Tailscale Inc & AUTHORS All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package archtest
+
+import (
+	"runtime"
+	"testing"
+
+	"inet.af/netstack/atomicbitops"
+)
+
+// tests netstack's AlignedAtomicInt64.
+func TestAlignedAtomicInt64(t *testing.T) {
+	type T struct {
+		A atomicbitops.AlignedAtomicInt64
+		x int32
+		B atomicbitops.AlignedAtomicInt64
+	}
+
+	t.Logf("I am %v/%v\n", runtime.GOOS, runtime.GOARCH)
+	var x T
+	x.A.Store(1)
+	x.B.Store(2)
+	if got, want := x.A.Load(), int64(1); got != want {
+		t.Errorf("A = %v; want %v", got, want)
+	}
+	if got, want := x.B.Load(), int64(2); got != want {
+		t.Errorf("A = %v; want %v", got, want)
+	}
+}