I thought AMD was basically the founder of 64-bti instructions in chips, so even a Sempron should work with LibreELEC => Sempron - Wikipedia
A bigger problem is likely the (onboard) graphics card.
I have a very early Sempron so not 64-bit. IIRC, both Intel and AMD were trying to extend the instruction sets (I am not that technical, so someone can correct my technical description faux-pas) at the same time. Both came out and everyone that mattered (except Intel) preferred the AMD implementation. After some understandable kicking and screaming, Intel agreed and implemented the AMD version for 64-bit. So not the founder, but they developed a better (faster? cheaper? simpler?) version.