Introduce runtime/platform directory for code shared between vm

runtime/vm/inttypes_support_win.h

Index: runtime/vm/inttypes_support_win.h
diff --git a/runtime/vm/inttypes_support_win.h b/runtime/vm/inttypes_support_win.h
deleted file mode 100644
index 6dc9b46d26f155e51ea045ba1120ab9acd9db2b9..0000000000000000000000000000000000000000
--- a/runtime/vm/inttypes_support_win.h
+++ /dev/null
@@ -1,17 +0,0 @@
-// Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
-// for details. All rights reserved. Use of this source code is governed by a
-// BSD-style license that can be found in the LICENSE file.
-
-#ifndef VM_INTTYPES_SUPPORT_WIN_H_
-#define VM_INTTYPES_SUPPORT_WIN_H_
-
-typedef signed __int8 int8_t;
-typedef signed __int16 int16_t;
-typedef signed __int32 int32_t;
-typedef signed __int64 int64_t;
-typedef unsigned __int8 uint8_t;
-typedef unsigned __int16 uint16_t;
-typedef unsigned __int32 uint32_t;
-typedef unsigned __int64 uint64_t;
-
-#endif  // VM_INTTYPES_SUPPORT_WIN_H_