It should be easy to adapt to other assemblers (it is likely that you won't need all the extra directives for the linker).
I think I got most of it from Kevin Thacker's [https://cpctech.cpcwiki.de/ The Unofficial Amstrad WWW Resource], but I can't find the exact link now. Anyway, Kevin is awesome and you should thank him if you have the chance.
.module crt0