Extend TF test coverage to unsupported targets.

src/compiler/arm64/linkage-arm64.cc

Index: src/compiler/arm64/linkage-arm64.cc
diff --git a/src/compiler/arm64/linkage-arm64.cc b/src/compiler/arm64/linkage-arm64.cc
index 05f80a08a330000baf428971f4fba8095fdfbd5f..72f6c55e0cc24195869350ddc210e5c0aa1f676c 100644
--- a/src/compiler/arm64/linkage-arm64.cc
+++ b/src/compiler/arm64/linkage-arm64.cc
@@ -14,6 +14,8 @@ namespace v8 {
 namespace internal {
 namespace compiler {
 
+#if V8_TURBOFAN_TARGET
+
 struct LinkageHelperTraits {
   static Register ReturnValueReg() { return x0; }
   static Register ReturnValue2Reg() { return x1; }
@@ -61,6 +63,8 @@ CallDescriptor* Linkage::GetSimplifiedCDescriptor(
   return LinkageHelper::GetSimplifiedCDescriptor<LinkageHelperTraits>(
       zone, num_params, return_type, param_types);
 }
+
+#endif  // V8_TURBOFAN_TARGET
 }
 }
 }  // namespace v8::internal::compiler