Support buildroots with alternative libcs

[berolinux]

We may need to build an OMV version with lower memory requirements (both for a project in Africa and for possible embedded systems) -- an obvious thing to do there is replace glibc with musl, libstdc++ with libc++ etc.

It would be nice to not need a separate platform for this so packages would always remain in sync.
Probably the easiest way to go about it is to add this as a separate architecture (e.g. have regular x86_64 and x86_64-musl -- like e.g. x86_64 and aarch64, pulling from the same sources but different binary repos).

rpm can already tell the difference between x86_64-linux and x86_64-linuxmusl, so that can be used to make sure packages like glibc and musl install to the right (different) place (/lib, /usr/include vs. crosscompile structure).