runtime: stub out weak pointer support

src/runtime/runtime.go

@@ -125,6 +125,14 @@ type Cleanup struct{}
 
 func (c Cleanup) Stop() {}
 
+//go:linkname registerWeakPointer weak.runtime_registerWeakPointer
+func registerWeakPointer(ptr unsafe.Pointer) unsafe.Pointer {
+	// TODO: unimplemented.
+	// I hope not implementing this won't break anything, like packages that
+	// expect weak pointers to be GC'd before they actually are.
+	return ptr
+}
+
 var godebugUpdate func(string, string)
 
 //go:linkname godebug_setUpdate internal/godebug.setUpdate