The Xash3D Github build guide shows you how to compile the Xash3D engine on a x86/x64 computer you need to know how to compile software that will be used on ARM computers, you can compile Xash3D using this guide I found on Reddit:
[Guide] Running Half-Life (and possibly other GoldSrc games) inside RetroPie!
https://www.reddit.com/r/RetroPie/comme ... r_goldsrc/
This guide is old not all instructions from this guide are necessary which will confuse most people if you don't know how to compile software on the RPI computer, this guide does produce a usable version of Xash3D at least on Raspbian Stretch I haven't compiled Xash3D on Raspbian Buster.