X87: Change num_double_registers() to num_allocatable_double_registers() for TestStackSlot.

  Although x87 has 8 registers, it use only 1 double register in TurboFan code generation for some limitations.

  So for TestStackSlot() function, use the num_allocatable_double_registers() to check the avaliable double registers
  of TurboFan is more suitable than num_double_registers().

BUG=

Review URL: https://codereview.chromium.org/1653913002

Cr-Commit-Position: refs/heads/master@{#33636}

test/cctest/compiler/test-run-native-calls.cc

@@ -1163,7 +1163,7 @@ void TestStackSlot(MachineType slot_type, T expected) {
   // function g from f which writes into the stack slot of f.
 
   if (RegisterConfiguration::ArchDefault(RegisterConfiguration::TURBOFAN)
-          ->num_double_registers() < 2)
+          ->num_allocatable_double_registers() < 2)
     return;
 
   Isolate* isolate = CcTest::InitIsolateOnce();