When the host OS (Fedora 27 in my case) features glibc 2.26. It's not possible to build older versions of glibc within OE-core. "rocko" features version 2.26 as well, so that will build fine. However morty and pyro will not build at all due to their libc version being older. See the error log for Pyro: https://pastebin.com/Dz1AiN82 Morty gives the exact same error.
You can work around this by adding: LOCALE_GENERATION_WITH_CROSS-LOCALEDEF_forcevariable = "0" in your local.conf, it will cause glibc-locale to show couple QA issues, but at least it will allow you to build the rest of the system before this issue is resolved.
Thank you, it works :)
Fixed in pyro: http://git.yoctoproject.org/cgit.cgi/poky/commit/?h=pyro&id=a75a2f4272226e924d8c9deb699a19ca9e606a5b and morty: http://git.yoctoproject.org/cgit.cgi/poky/commit/?h=morty&id=8c84d07063ea0ebe8f335fafc2e619555bdab812