Refactors CPU feature detection.

runtime/vm/cpu.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 VM_CPU_H_
 #define VM_CPU_H_
 
 #include "vm/allocation.h"
 
 namespace dart {
 
 // Forward Declarations.
 class Error;
 class Instance;
 
 
 class CPU : public AllStatic {
  public:
   static void FlushICache(uword start, uword size);
   static const char* Id();
 };
 
+
 }  // namespace dart
 
+#if defined(TARGET_ARCH_IA32)
+#include "vm/cpu_ia32.h"
+#elif defined(TARGET_ARCH_X64)
+#include "vm/cpu_x64.h"
+#elif defined(TARGET_ARCH_ARM)
+#include "vm/cpu_arm.h"
+#elif defined(TARGET_ARCH_MIPS)
+#include "vm/cpu_mips.h"
+#else
+#error Unknown architecture.
+#endif
+
 #endif  // VM_CPU_H_