Preliminary changes for PowerPC 64-bits support

autoconf and libyaml needed updated config.sub and config.guess files
on ppc64le.

gnu-config component was created to centralize updated config.sub and
config.guess files. autoconf and libyaml now extend gnu-config and
overwrite their bundled config.guess and config.sub with gnu-config's.

in the future, any component can extend gnu-config to update it's own
config.sub and config.guess files if necessary.