Fix/implement new write barrier for x64.

src/write-buffer.h

Index: src/write-buffer.h
===================================================================
--- src/write-buffer.h	(revision 6184)
+++ src/write-buffer.h	(working copy)
@@ -41,20 +41,12 @@
 // between spaces.
 class WriteBuffer : public AllStatic {
  public:
-  static Address TopAddress() { return reinterpret_cast<Address>(&top_); }
+  static inline Address TopAddress();
 
   static void Setup();
   static void TearDown();
 
-  static inline void Mark(Address addr) {
-    *top_++ = addr;
-    if ((reinterpret_cast<uintptr_t>(top_) & kWriteBufferOverflowBit) != 0) {
-      ASSERT(top_ == limit_);
-      Compact();
-    } else {
-      ASSERT(top_ < limit_);
-    }
-  }
+  static inline void Mark(Address addr);
 
   static const int kWriteBufferOverflowBit = 1 << 16;
   static const int kWriteBufferSize = kWriteBufferOverflowBit;
@@ -64,7 +56,6 @@
   static void Compact();
 
  private:
-  static Address* top_;
   static Address* start_;
   static Address* limit_;
   static VirtualMemory* virtual_memory_;