remove most uses of Isolate::Current in arch specific files

src/x64/code-stubs-x64.h

Index: src/x64/code-stubs-x64.h
diff --git a/src/x64/code-stubs-x64.h b/src/x64/code-stubs-x64.h
index e430bf2c80540350ea61dc9edd46f9635f9b659e..41678ecd20e0aac9ee7b7862551de3bf0f86758a 100644
--- a/src/x64/code-stubs-x64.h
+++ b/src/x64/code-stubs-x64.h
@@ -69,7 +69,7 @@ class StoreBufferOverflowStub: public PlatformCodeStub {
 
   void Generate(MacroAssembler* masm);
 
-  virtual bool IsPregenerated() { return true; }
+  virtual bool IsPregenerated(Isolate* isolate) V8_OVERRIDE { return true; }
   static void GenerateFixedRegStubsAheadOfTime(Isolate* isolate);
   virtual bool SometimesSetsUpAFrame() { return false; }
 
@@ -321,7 +321,7 @@ class RecordWriteStub: public PlatformCodeStub {
     INCREMENTAL_COMPACTION
   };
 
-  virtual bool IsPregenerated();
+  virtual bool IsPregenerated(Isolate* isolate) V8_OVERRIDE;
   static void GenerateFixedRegStubsAheadOfTime(Isolate* isolate);
   virtual bool SometimesSetsUpAFrame() { return false; }