tailcfg, control/controlclient: add GoArch, populate OSVersion on Linux

tailcfg/tailcfg.go

@@ -260,6 +260,7 @@ type Hostinfo struct {
 	OSVersion     string       // operating system version, with optional distro prefix ("Debian 10.4", "Windows 10 Pro 10.0.19041")
 	DeviceModel   string       // mobile phone model ("Pixel 3a", "iPhone 11 Pro")
 	Hostname      string       // name of the host the client runs on
+	GoArch        string       // the host's GOARCH value (of the running binary)
 	RoutableIPs   []wgcfg.CIDR `json:",omitempty"` // set of IP ranges this client can route
 	RequestTags   []string     `json:",omitempty"` // set of ACL tags this node wants to claim
 	Services      []Service    `json:",omitempty"` // services advertised by this machine

tailcfg/tailcfg_test.go

@@ -24,7 +24,7 @@ func fieldsOf(t reflect.Type) (fields []string) {
 func TestHostinfoEqual(t *testing.T) {
 	hiHandles := []string{
 		"IPNVersion", "FrontendLogID", "BackendLogID", "OS", "OSVersion",
-		"DeviceModel", "Hostname", "RoutableIPs", "RequestTags", "Services",
+		"DeviceModel", "Hostname", "GoArch", "RoutableIPs", "RequestTags", "Services",
 		"NetInfo",
 	}
 	if have := fieldsOf(reflect.TypeOf(Hostinfo{})); !reflect.DeepEqual(have, hiHandles) {