rbinstall.rb: make stable files in .gemspec

* tool/rbinstall.rb (Gem::Specification#collect): make stable
  Gem::Specification.files in default .gemspecs the different order of
  "files" in .gemspec files makes them different therefore possibly
  conflicting in multilib scenario.  patch by vo.x (Vit Ondruch) at
  [ruby-core:57544] [Bug #8623].

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@43587 b2dd03c8-39d4-4d8f-98ff-823fe69b080e