Include "ruby2_keywords.rb" in $LOADED_FEATURES like MRI

* Hardcoded instead of a real file to pass this test:
  https://github.com/ruby/ruby2_keywords/blob/64aad913e16d7e6008aa6ca06cf3f1b6fa864c4a/test/test_keyword.rb#L15

Author: eregon

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

@@ -34,6 +34,9 @@ def self.define_read_only_global(name, getter)
     LOAD_PATH = Truffle::SynchronizedArray.new
     LOADED_FEATURES = Truffle::SynchronizedArray.new
 
+    # Always provided features: ruby --disable-gems -e 'puts $"'
+    LOADED_FEATURES << 'ruby2_keywords.rb'
+
     define_read_only_global(:$LOAD_PATH, -> { LOAD_PATH })
     define_read_only_global(:$LOADED_FEATURES, -> { LOADED_FEATURES })
 

src/main/ruby/truffleruby/post-boot/post-boot.rb

@@ -46,6 +46,8 @@
         if path.start_with?(old_home)
           path.replace Truffle::Ropes.flatten_rope(path[old_home.size..-1])
           paths_starting_with_home << path
+        elsif !path.include?('/')
+          # relative path for always provided features like 'ruby2_keywords.rb'
         else
           raise "Path #{path.inspect} in $LOAD_PATH or $LOADED_FEATURES was expected to start with #{old_home}"
         end