Update config.{guess,sub}

Autoconf hasn’t had an update since 2014, and it doesn’t look like it will soon[1]
This updates config.{guess,sub}

It basically does exactly what the config.guess script says:
> It is advised that you download the most up to date version of the config scripts from

It adds support for e.g. `-ios`, which allows to have targets like `aarch64-apple-ios`.

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[1]: http://lists.gnu.org/archive/html/autoconf/2016-07/msg00017.html