LLVM isn't completely platform independent (e.g. all integers are fixed-width, so the frontend needs to know the pointer size to emit pointer-sized integers) an

LLVM isn't completely platform independent (e.g. all integers are fixed-width, so the frontend needs to know the pointer size to emit pointer-sized integers) and there are some configuration options to be passed down to LLVM/the linker. Additionally, it is useful for the front-end to understand things like the layout of types, and the standard library usually also needs changes to paper over any platform differences (although I imagine AVR doesn't need many, mostly in the compiler-support library compiler-rt, given not much of the standard library will work).