mruby-sprintf: follow CRuby changes

See [r46656](https://github.com/ruby/ruby/commit/3659c10b9d156cf8e12602996aa4719efc437ecd), [r46657](https://github.com/ruby/ruby/commit/420523389ec582f13924608ceb7640c95d7dd7a4), [r46658](https://github.com/ruby/ruby/commit/3f48c1fdf785f0d5a9939fc6163d9015d40e2b79), [r46659](https://github.com/ruby/ruby/commit/18a9926ad992f166cd1287ded00e9b7929c897c3) and [r46756](https://github.com/ruby/ruby/commit/8b427e357b707131614a45a3746dd06f9ac9a14b).