Merge pull request #654 from visualfc/fixstd

cpp/std: fix std::string def

cpp/std/string.go

@@ -31,7 +31,7 @@ type StringView = string
 
 // String represents a C++ std::string object.
 type String struct {
-	Unused [3]uintptr
+	Unused [3 * unsafe.Sizeof(0)]byte
 }
 
 // llgo:link (*String).InitEmpty C.stdStringInitEmpty