set top level folder to same as archname of tarball

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@46558 b2dd03c8-39d4-4d8f-98ff-823fe69b080e

Author: hone

tool/make-snapshot

@@ -203,7 +203,9 @@ def package(rev, destdir)
     tag ||= "r#{revision}"
   end
   unless v == $exported
-    if tag.empty?
+    if $archname
+      n = $archname
+    elsif tag.empty?
       n = "ruby-#{version}"
     else
       n = "ruby-#{version}-#{tag}"