Avoid loading RubyGems in irb/completion.rb

* Fixes spec/truffle/lazy_rubygems_spec.rb

Author: eregon

lib/mri/irb/completion.rb

@@ -69,7 +69,7 @@ def self.absolute_path?(p) # TODO Remove this method after 2.6 EOL.
     end
 
     GEM_PATHS =
-      if defined?(Gem::Specification)
+      if !defined?(::TruffleRuby) and defined?(Gem::Specification)
         Gem::Specification.latest_specs(true).map { |s|
           s.require_paths.map { |p|
             if absolute_path?(p)