distroregistry: Add host's architecture name to the registry

The store needs to know what default distribution and arch to use for
images built with a blueprint that has no distro set. Store the host's
architecture name along side the host's distribution details.

SetHostArchName() allows this host arch to be overridden for testing,
which uses arch names that do not match any host.

internal/distroregistry/distroregistry.go

@@ -40,14 +40,16 @@ type supportedDistro struct {
 }
 
 type Registry struct {
-	distros    map[string]distro.Distro
-	hostDistro distro.Distro
+	distros      map[string]distro.Distro
+	hostDistro   distro.Distro
+	hostArchName string
 }
 
 func New(hostDistro distro.Distro, distros ...distro.Distro) (*Registry, error) {
 	reg := &Registry{
-		distros:    make(map[string]distro.Distro),
-		hostDistro: hostDistro,
+		distros:      make(map[string]distro.Distro),
+		hostDistro:   hostDistro,
+		hostArchName: common.CurrentArch(),
 	}
 	for _, d := range distros {
 		name := d.Name()
@@ -136,3 +138,13 @@ func mangleHostDistroName(name string, isBeta, isStream bool) string {
 func (r *Registry) FromHost() distro.Distro {
 	return r.hostDistro
 }
+
+// HostArchName returns the host's arch name
+func (r *Registry) HostArchName() string {
+	return r.hostArchName
+}
+
+// SetHostArchName can be used to override the host's arch name for testing
+func (r *Registry) SetHostArchName(name string) {
+	r.hostArchName = name
+}