Preliminary changes for PowerPC 64-bits support

These changes allow to build SlapOS on Debian Buster ppc64el.

The PowerPC 64-bits little endian platform appeared more recently into autoconf and therefore needed updated `config.sub` and `config.guess` files.

Somehow these only needed to be patched on autoconf itself which is quite dated (2012), and libyaml that does not ship it's own `config.sub` and `config.guess` files. Other components built fine out of the box.

`config.sub` and `config.guess` files were taken from Debian Buster itself. More information at: https://wiki.debian.org/AutoTools/autoconf

**This merge request would have to be backported to previous releases in the case components reference older releases of SlapOS, which, if I understand correctly, is the case some times.**

See merge request nexedi/slapos!780