Only build ARMv5 (fixes #1218)

With this change, the build system only builds one ARM variant - ARMv5.
We call the build architecture simply "arm", as this is what
runtime.GOARCH says.

internal/upgrade/upgrade_common.go

@@ -18,6 +18,8 @@ package upgrade
 
 import (
 	"errors"
+	"fmt"
+	"runtime"
 	"strconv"
 	"strings"
 
@@ -225,3 +227,12 @@ func versionParts(v string) ([]int, []interface{}) {
 
 	return release, prerelease
 }
+
+func releaseName(tag string) string {
+	switch runtime.GOOS {
+	case "darwin":
+		return fmt.Sprintf("syncthing-macosx-%s-%s.", runtime.GOARCH, tag)
+	default:
+		return fmt.Sprintf("syncthing-%s-%s-%s.", runtime.GOOS, runtime.GOARCH, tag)
+	}
+}