Add a TurboFan skeleton for StringAddStub.

src/compiler/ppc/code-generator-ppc.cc

Index: src/compiler/ppc/code-generator-ppc.cc
diff --git a/src/compiler/ppc/code-generator-ppc.cc b/src/compiler/ppc/code-generator-ppc.cc
index bdebd304c04495d1abc176aa75aa6bf921315bd4..d7b038394d937df00b565316c08ecec98352f597 100644
--- a/src/compiler/ppc/code-generator-ppc.cc
+++ b/src/compiler/ppc/code-generator-ppc.cc
@@ -1284,7 +1284,7 @@ void CodeGenerator::AssemblePrologue() {
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
@@ -1330,7 +1330,7 @@ void CodeGenerator::AssembleReturn() {
     }
     __ LeaveFrame(StackFrame::MANUAL);
     __ Ret();
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     int pop_count = descriptor->IsJSFunctionCall()
                         ? static_cast<int>(descriptor->JSParameterCount())
                         : 0;