Introduce runtime/platform directory for code shared between vm

runtime/vm/c99_support_win.h

-// 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_C99_SUPPORT_WIN_H_
-#define VM_C99_SUPPORT_WIN_H_
-
-// Visual C++ is missing a bunch of C99 math macros and
-// functions. Define them here.
-
-#include <float.h>
-
-static const unsigned __int64 kQuietNaNMask =
-    static_cast<unsigned __int64>(0xfff) << 51;
-
-#define INFINITY HUGE_VAL
-#define NAN \
-    *reinterpret_cast<const double*>(&kQuietNaNMask)
-
-static inline int isinf(double x) {
-  return (_fpclass(x) & (_FPCLASS_PINF | _FPCLASS_NINF)) != 0;
-}
-
-static inline int isnan(double x) {
-  return _isnan(x);
-}
-
-static inline int signbit(double x) {
-  if (x == 0) {
-    return _fpclass(x) & _FPCLASS_NZ;
-  } else {
-    return x < 0;
-  }
-}
-
-static inline double trunc(double x) {
-  if (x < 0) {
-    return ceil(x);
-  } else {
-    return floor(x);
-  }
-}
-
-static inline double round(double x) {
-  if (x < 0) {
-    return ceil(x - 0.5);
-  } else {
-    return floor(x + 0.5);
-  }
-}
-
-#endif  // VM_C99_SUPPORT_WIN_H_