[rubygems/rubygems] Add `Gem::Specification#gem_dir` back

If old Bundler versions that unconditionally try to remove this method
are run with RubyGems versions _without_ this method, Bundler crashes
because it tries to remove a method that does not exist.

We need to wait until RubyGems cannot install any Bundler versions that
unconditionally remove this method.

https://github.com/rubygems/rubygems/commit/98804d261d

lib/rubygems/specification.rb

@@ -1912,6 +1912,14 @@ class Gem::Specification < Gem::BasicSpecification
     @full_name ||= super
   end
 
+  ##
+  # Work around old bundler versions removing my methods
+  # Can be removed once RubyGems can no longer install Bundler 2.5
+
+  def gem_dir # :nodoc:
+    super
+  end
+
   def gems_dir
     @gems_dir ||= File.join(base_dir, "gems")
   end