Even though the RX 480 is not a very recent GPU, the 8 GB VRAM are quite decent and make it possible to run LLMs much faster than with the CPU.
To make it running, three steps are necessary: Installing vulkan, building shaderc and finally building lama.cpp including the vulkan backend (assuming git, a c compiler, cmake, ninja, etc. are installed on a sufficiently recent Ubuntu).
Run sudo apt install libvulkan-dev vulkan-tools glslang-tools
.