[FR] add a kernel demo in host project

[glyh]

I am struggling to find working version of a kernel hello world. There's https://github.com/andrewrk/HellOS by andrew but the code is outdated and the new PR doesn't work on my computer as well.

I think it make sense to maintain a kernel hello world demo as test in zig's language repo as one of the goal of the language is to be a good language to write low-level code. OS is obvious a nontrivial and related field here.