atomic requires struct field to be 64-bit aligned

fix https://github.com/spf13/hugo/issues/1621

memfile.go

@@ -34,8 +34,11 @@ type MemDir interface {
 }
 
 type InMemoryFile struct {
-	sync.Mutex
+	// atomic requires 64-bit alignment for struct field access
 	at           int64
+	readDirCount int64
+
+	sync.Mutex
 	name    string
 	data    []byte
 	memDir  MemDir
@@ -43,7 +46,6 @@ type InMemoryFile struct {
 	closed  bool
 	mode    os.FileMode
 	modtime time.Time
-	readDirCount int64
 }
 
 func MemFileCreate(name string) *InMemoryFile {