tool/unicode_norm_gen.rb: Fixed escaping of backslash and
double quote ('\\\&' -> "\\\\\\\&"; double quoted string
is needed to make \& mean last match; double double
backslashes are needed because of two layers of escaping).

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@48089 b2dd03c8-39d4-4d8f-98ff-823fe69b080e