For the language, i think assembly is a no-no with cabal, as such it need to be embedded in gcc inline assembly if you want to have something that works (unless there's a secret way to run assembler in a portable fashion in cabal).
I don't know if cabal knows this, but assembler files with .s (and maybe .asm on Windows?) extension are recognized by most C compilers and handed off to the assembler; as such, simply augmenting cabal's C rules with those extensions should be sufficient.