variable.c (struct global_variable): shrink by 8 bytes on 64-bit

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45295 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
* 2014-03-09

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45296 b2dd03c8-39d4-4d8f-98ff-823fe69b080e