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

src/mips/interface-descriptors-mips.cc

Index: src/mips/interface-descriptors-mips.cc
diff --git a/src/mips/interface-descriptors-mips.cc b/src/mips/interface-descriptors-mips.cc
index 7edc606dc5586fb7c423ac065863b3343590234e..2f6bf5ce6285f4865612311b4d6e22084ff2380c 100644
--- a/src/mips/interface-descriptors-mips.cc
+++ b/src/mips/interface-descriptors-mips.cc
@@ -99,6 +99,10 @@ void ToNumberDescriptor::InitializePlatformSpecific(
 
 
 // static
+const Register ToStringDescriptor::ReceiverRegister() { return a0; }
+
+
+// static
 const Register ToObjectDescriptor::ReceiverRegister() { return a0; }