kGcnArch query is no-op

Author: Peter Yeh

src/runtime/metal/metal_device_api.mm

@@ -63,6 +63,7 @@
     case kMultiProcessorCount: return;
     case kMaxThreadDimensions: return;
     case kExist: break;
+    case kGcnArch: return; 
   }
 }
 

src/runtime/opencl/opencl_device_api.cc

@@ -114,6 +114,7 @@ void OpenCLWorkspace::GetAttr(
       *rv = ss.str();
       break;
     }
+    case kGcnArch: return;
   }
 }
 

src/runtime/vulkan/vulkan.cc

@@ -398,6 +398,8 @@ void VulkanDeviceAPI::GetAttr(TVMContext ctx, DeviceAttrKind kind, TVMRetValue*
       break;
     case kMaxThreadDimensions:
       break;
+    case kGcnArch:
+      return;
   }
 }