runtime: add missing arch1_arm64.go

Missed this one from the bitbucket migration.

Author: davecheney

src/runtime/arch1_arm64.go

@@ -0,0 +1,15 @@
+// Copyright 2011 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 runtime
+
+const (
+	thechar           = '7'
+	_BigEndian        = 0
+	_CacheLineSize    = 32
+	_RuntimeGogoBytes = 60
+	_PhysPageSize     = 65536*goos_nacl + 4096*(1-goos_nacl)
+	_PCQuantum        = 4
+	_Int64Align       = 8
+)