[es6] Add a flag to the Isolate that defines whether ES2015 tail call elimination is enabled or not.

src/assembler.h

Index: src/assembler.h
diff --git a/src/assembler.h b/src/assembler.h
index b6c48cf46f65f59244b9ae3f86262a0b8e903c8c..8dc528547cc63c41c277e919c8b30ed6b210ef92 100644
--- a/src/assembler.h
+++ b/src/assembler.h
@@ -1029,6 +1029,9 @@ class ExternalReference BASE_EMBEDDED {
 
   static ExternalReference cpu_features();
 
+  static ExternalReference is_tail_call_elimination_enabled_address(
+      Isolate* isolate);
+
   static ExternalReference debug_is_active_address(Isolate* isolate);
   static ExternalReference debug_after_break_target_address(Isolate* isolate);