Add x64 implementation test support for assembler and code generator.

src/x64/macro-assembler-x64.h

Index: src/x64/macro-assembler-x64.h
===================================================================
--- src/x64/macro-assembler-x64.h	(revision 2140)
+++ src/x64/macro-assembler-x64.h	(working copy)
@@ -67,6 +67,16 @@
   MacroAssembler(void* buffer, int size);
 
   // ---------------------------------------------------------------------------
+  // x64 Implementation Support
+
+  // Test the MacroAssembler by constructing and calling a simple JSFunction.
+  // Cannot be done using API because this must be done in the middle of the
+  // bootstrapping process.
+  // TODO(X64): Remove once we can get through the bootstrapping process.
+
+  static void ConstructAndTestJSFunction();
+
+  // ---------------------------------------------------------------------------
   // GC Support
 
   // Set the remembered set bit for [object+offset].