Config double register number as 1 for X87 Turbofan.

src/x87/assembler-x87.h

Index: src/x87/assembler-x87.h
diff --git a/src/x87/assembler-x87.h b/src/x87/assembler-x87.h
index ea05ab975f03c02eb37a195606d6c7c8ae46c12b..61faab6cb0e9a10a4ec4f4091744770314be8c71 100644
--- a/src/x87/assembler-x87.h
+++ b/src/x87/assembler-x87.h
@@ -151,12 +151,18 @@ struct X87Register {
     return kMaxNumAllocatableRegisters;
   }
 
-
   // TODO(turbofan): Proper support for float32.
   static int NumAllocatableAliasedRegisters() {
     return NumAllocatableRegisters();
   }
 
+  static int NumAllocatableRegistersForTurbo() {
+    return 1;
+  }
+
+  static int NumAllocatableAliasedRegistersForTurbo() {
+    return NumAllocatableRegistersForTurbo();
+  }
 
   static int ToAllocationIndex(X87Register reg) {
     return reg.code_;