cmd/syncthing: Enable KCP by default

Also, use upstream library, as my changes have been merged.

vendor/github.com/templexxx/cpufeat/cpu_mips.go

@@ -0,0 +1,7 @@
+// Copyright 2017 The Go Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style
+// license that can be found in the LICENSE file.
+
+package cpufeat
+
+const CacheLineSize = 32