ipn/store: omit AWS & Kubernetes support on 'small' Linux GOARCHes

This removes AWS and Kubernetes support from Linux binaries by default
on GOARCH values where people don't typically run on AWS or use
Kubernetes, such as 32-bit mips CPUs.

It primarily focuses on optimizing for the static binaries we
distribute. But for people building it themselves, they can set
ts_kube or ts_aws (the opposite of ts_omit_kube or ts_omit_aws) to
force it back on.

Makes tailscaled binary ~2.3MB (~7%) smaller.

Updates #7272, #10627 etc

Change-Id: I42a8775119ce006fa321462cb2d28bc985d1c146
Signed-off-by: Brad Fitzpatrick <bradfitz@tailscale.com>

ipn/store/store_kube.go

@@ -0,0 +1,25 @@
+// Copyright (c) Tailscale Inc & AUTHORS
+// SPDX-License-Identifier: BSD-3-Clause
+
+//go:build (ts_kube || (linux && (arm64 || amd64))) && !ts_omit_kube
+
+package store
+
+import (
+	"strings"
+
+	"tailscale.com/ipn"
+	"tailscale.com/ipn/store/kubestore"
+	"tailscale.com/types/logger"
+)
+
+func init() {
+	registerAvailableExternalStores = append(registerAvailableExternalStores, registerKubeStore)
+}
+
+func registerKubeStore() {
+	Register("kube:", func(logf logger.Logf, path string) (ipn.StateStore, error) {
+		secretName := strings.TrimPrefix(path, "kube:")
+		return kubestore.New(logf, secretName)
+	})
+}