Add number allocation call to runtime.cc instead of misusing

src/x64/full-codegen-x64.cc

Index: src/x64/full-codegen-x64.cc
===================================================================
--- src/x64/full-codegen-x64.cc	(revision 5036)
+++ src/x64/full-codegen-x64.cc	(working copy)
@@ -2243,11 +2243,8 @@
   __ jmp(&heapnumber_allocated);
 
   __ bind(&slow_allocate_heapnumber);
-  // To allocate a heap number, and ensure that it is not a smi, we
-  // call the runtime function FUnaryMinus on 0, returning the double
-  // -0.0.  A new, distinct heap number is returned each time.
-  __ Push(Smi::FromInt(0));
-  __ CallRuntime(Runtime::kNumberUnaryMinus, 1);
+  // Allocate a heap number.
+  __ CallRuntime(Runtime::kNumberAlloc, 0);
   __ movq(rbx, rax);
 
   __ bind(&heapnumber_allocated);