template/unicode_norm_gen.tmpl: from tool/unicode_norm_gen.rb

* template/unicode_norm_gen.tmpl: use generic_erb.rb to update if
  changed and manage timestamp, so that source tree on read-only
  filesystem works.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48129 b2dd03c8-39d4-4d8f-98ff-823fe69b080e