Raise the correct error for frozen $LOADED_FEATURES with pre-initialization

* A simple test is `$LOADED_FEATURES.freeze; require "ostruct"`,
  part of MRI tests.

Author: eregon

src/main/ruby/truffleruby/core/truffle/feature_loader.rb

@@ -281,6 +281,7 @@ def self.with_synchronized_features
     # always called inside #with_synchronized_features
     def self.get_loaded_features_index
       unless @loaded_features_version == $LOADED_FEATURES.version
+        raise '$LOADED_FEATURES is frozen; cannot append feature' if $LOADED_FEATURES.frozen?
         @loaded_features_index.clear
         $LOADED_FEATURES.map! do |val|
           val = StringValue(val)