Add a TurboFan skeleton for StringAddStub.

src/compiler/arm64/code-generator-arm64.cc

Index: src/compiler/arm64/code-generator-arm64.cc
diff --git a/src/compiler/arm64/code-generator-arm64.cc b/src/compiler/arm64/code-generator-arm64.cc
index 775f519519a0df93089cc1da5c17f7dbcf14e8ae..4c05eb36b1b23066806070fd906c55d1dc4ea48a 100644
--- a/src/compiler/arm64/code-generator-arm64.cc
+++ b/src/compiler/arm64/code-generator-arm64.cc
@@ -1078,7 +1078,7 @@ void CodeGenerator::AssemblePrologue() {
     __ Prologue(info->IsCodePreAgingActive());
     frame()->SetRegisterSaveAreaSize(
         StandardFrameConstants::kFixedFrameSizeFromFp);
-  } else if (stack_slots > 0) {
+  } else if (needs_frame_) {
     __ SetStackPointer(jssp);
     __ StubPrologue();
     frame()->SetRegisterSaveAreaSize(
@@ -1127,7 +1127,7 @@ void CodeGenerator::AssembleReturn() {
     __ Mov(csp, fp);
     __ Pop(fp, lr);
     __ Ret();
-  } else if (descriptor->IsJSFunctionCall() || stack_slots > 0) {
+  } else if (descriptor->IsJSFunctionCall() || needs_frame_) {
     __ Mov(jssp, fp);
     __ Pop(fp, lr);
     int pop_count = descriptor->IsJSFunctionCall()