VM: Move calling convention-dependent code out of cc tests.

runtime/vm/object_x64_test.cc

Index: runtime/vm/object_x64_test.cc
diff --git a/runtime/vm/object_x64_test.cc b/runtime/vm/object_x64_test.cc
index 639d8d0ac01e4e559babb8bd2cbd15a9a81cb317..e5b1033ee83478a7246361735f03cf5429423781 100644
--- a/runtime/vm/object_x64_test.cc
+++ b/runtime/vm/object_x64_test.cc
@@ -34,10 +34,9 @@ void GenerateIncrement(Assembler* assembler) {
 void GenerateEmbedStringInCode(Assembler* assembler, const char* str) {
   const String& string_object =
       String::ZoneHandle(String::New(str, Heap::kOld));
-  __ pushq(PP);  // Save caller's pool pointer and load a new one here.
-  __ LoadPoolPointer();
+  __ EnterStubFrame();
   __ LoadObject(RAX, string_object);
-  __ popq(PP);  // Restore caller's pool pointer.
+  __ LeaveStubFrame();
   __ ret();
 }