make-snapshot: support xz

* tool/make-snapshot (package): support xz.  no longer runs with
  1.8 or earlier.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45934 b2dd03c8-39d4-4d8f-98ff-823fe69b080e
make-snapshot: SHA512

* tool/make-snapshot (package): add SHA512.

git-svn-id: svn+ssh://ci.ruby-lang.org/ruby/trunk@45935 b2dd03c8-39d4-4d8f-98ff-823fe69b080e