Add a MathFloor stub generated with TurboFan

src/interface-descriptors.h

Index: src/interface-descriptors.h
diff --git a/src/interface-descriptors.h b/src/interface-descriptors.h
index 6ce5faacf44df98d317ded079e1e2d5f2483372b..19f847559216d01730ccdd383f39ef96d805d07b 100644
--- a/src/interface-descriptors.h
+++ b/src/interface-descriptors.h
@@ -57,6 +57,7 @@ class PlatformInterfaceDescriptor;
   V(StoreArrayLiteralElement)                 \
   V(MathPowTagged)                            \
   V(MathPowInteger)                           \
+  V(MathRoundVariant)                         \
   V(ContextOnly)                              \
   V(GrowArrayElements)
 
@@ -521,6 +522,12 @@ class MathPowIntegerDescriptor : public CallInterfaceDescriptor {
 };
 
 
+class MathRoundVariantDescriptor : public CallInterfaceDescriptor {
+ public:
+  DECLARE_DESCRIPTOR(MathRoundVariantDescriptor, CallInterfaceDescriptor)
+};
+
+
 class ContextOnlyDescriptor : public CallInterfaceDescriptor {
  public:
   DECLARE_DESCRIPTOR(ContextOnlyDescriptor, CallInterfaceDescriptor)