make-snapshot: fix gems update

* tool/make-snapshot (package): call system and IO.popen with
  arguments array, add macros NULLCMD and RUNRUBY to update gems,
  and not subsitute string interpolations which are used in
  update-gems.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48122 b2dd03c8-39d4-4d8f-98ff-823fe69b080e