[runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

src/interface-descriptors.cc

Index: src/interface-descriptors.cc
diff --git a/src/interface-descriptors.cc b/src/interface-descriptors.cc
index 913847bc13a6aebf6706cf75c62d264c39987d75..e9bafafba95ec1a7d4b1ad05aad154f4dc8da2c7 100644
--- a/src/interface-descriptors.cc
+++ b/src/interface-descriptors.cc
@@ -162,6 +162,13 @@ void InstanceOfDescriptor::InitializePlatformSpecific(
 }
 
 
+void ToStringDescriptor::InitializePlatformSpecific(
+    CallInterfaceDescriptorData* data) {
+  Register registers[] = {ReceiverRegister()};
+  data->InitializePlatformSpecific(arraysize(registers), registers);
+}
+
+
 void ToObjectDescriptor::InitializePlatformSpecific(
     CallInterfaceDescriptorData* data) {
   Register registers[] = {ReceiverRegister()};