rbinstall.rb: gem subdirectories

* tool/rbinstall.rb (gem): Gem.ensure_gem_subdirectories makes
  subdirectories group-writable, so make them with $dir_mode.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@38870 b2dd03c8-39d4-4d8f-98ff-823fe69b080e