* nacl/nacl-config.rb: Use File.exist? instead of executable?
  for irt_core. Recent nacl_sdk has non-executable irt_core.
  Patch by Shinichiro Hamaji.
  [Fixes GH-529] https://github.com/ruby/ruby/pull/529

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46529 b2dd03c8-39d4-4d8f-98ff-823fe69b080e