Since macOS 13, CFString family API used in
`rb_str_append_normalized_ospath` may internally use Objective-C classes
(`NSTaggedPointerString` and `NSPlaceholderMutableString`) for small strings.

On the other hand, Objective-C classes should not be used for the first
time in a `fork()`'ed but not `exec()`'ed process. Violations for this rule
can result deadlock during class initialization, so Objective-C runtime
conservatively crashes on such cases by default.

Therefore, we need to use CFString API to initialize Objective-C classes
used internally *before* `fork()`.

For more details, see https://bugs.ruby-lang.org/issues/18912