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

src/assembler.cc

Index: src/assembler.cc
diff --git a/src/assembler.cc b/src/assembler.cc
index 2f0b8805977771521aa4bfab070a77c4d3308bd4..e9ecf90ecebc41912984e789c32433b49b5a92d3 100644
--- a/src/assembler.cc
+++ b/src/assembler.cc
@@ -1632,6 +1632,10 @@ ExternalReference ExternalReference::cpu_features() {
   return ExternalReference(&CpuFeatures::supported_);
 }
 
+ExternalReference ExternalReference::is_tail_call_elimination_enabled_address(
+    Isolate* isolate) {
+  return ExternalReference(isolate->is_tail_call_elimination_enabled_address());
+}
 
 ExternalReference ExternalReference::debug_is_active_address(
     Isolate* isolate) {