Add .clang-format and run clang-format on runtime/platform.

runtime/platform/floating_point.h

 // Copyright (c) 2012, 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 RUNTIME_PLATFORM_FLOATING_POINT_H_
 #define RUNTIME_PLATFORM_FLOATING_POINT_H_
 
-inline double fmod_ieee(double x, double y) { return fmod(x, y); }
-inline double atan2_ieee(double y, double x) { return atan2(y, x); }
+inline double fmod_ieee(double x, double y) {
+  return fmod(x, y);
+}
+inline double atan2_ieee(double y, double x) {
+  return atan2(y, x);
+}
 
 #endif  // RUNTIME_PLATFORM_FLOATING_POINT_H_