scripts: config.guess: update to 2021-05-24

This script hasn't seen an update in multiple years, update it to the
latest version provided upstream. Both `config.guess` and `config.sub`
are copied from upstream[1] and not modified.

The full changelog is available within the upstream repository[1].

[1]: https://git.savannah.gnu.org/git/config.git

Signed-off-by: Paul Spooren <[email protected]>