Make RbConfig::CONFIG values mutable [Bug #16738]
As `RbConfig.expand` modifies the argument and involved `CONFIG`
values, its values should be mutable.
git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/branches/ruby_2_5@67872 b2dd03c8-39d4-4d8f-98ff-823fe69b080e