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

src/arm64/interface-descriptors-arm64.cc

Index: src/arm64/interface-descriptors-arm64.cc
diff --git a/src/arm64/interface-descriptors-arm64.cc b/src/arm64/interface-descriptors-arm64.cc
index 8adb483222c0723a0e68a7fc52333a568518bc5d..632b96b8aaaaa2e828a88b7bc1fbebc82142d81e 100644
--- a/src/arm64/interface-descriptors-arm64.cc
+++ b/src/arm64/interface-descriptors-arm64.cc
@@ -100,6 +100,10 @@ void ToNumberDescriptor::InitializePlatformSpecific(
 
 
 // static
+const Register ToStringDescriptor::ReceiverRegister() { return x0; }
+
+
+// static
 const Register ToObjectDescriptor::ReceiverRegister() { return x0; }