X87: [runtime] Add %ToString and %_ToString and remove the TO_STRING builtin.

src/x87/interface-descriptors-x87.cc

Index: src/x87/interface-descriptors-x87.cc
diff --git a/src/x87/interface-descriptors-x87.cc b/src/x87/interface-descriptors-x87.cc
index d68802a19879c1f98ede637bd0a476b80cd0ec50..3ffccd3b712c5f9a11198cb7f1d702cc87e71cf1 100644
--- a/src/x87/interface-descriptors-x87.cc
+++ b/src/x87/interface-descriptors-x87.cc
@@ -107,6 +107,10 @@ void ToNumberDescriptor::InitializePlatformSpecific(
 
 
 // static
+const Register ToStringDescriptor::ReceiverRegister() { return eax; }
+
+
+// static
 const Register ToObjectDescriptor::ReceiverRegister() { return eax; }