Russell King さんが次のように書いてます。
ARM doesn't require any special API modification to call functions from assembly. x86 has two different calling conventions - one is to stack all function arguments, the other passes some arguments via registers. asmlinkage selects between them. However, we keep the asmlinkage labelling on ARM because it's useful to indicate which functions are called by assembly - and therefore which need closer inspection when their signature is changed to make sure the assembly calling them is also fixed.
要は x86 で意味を持つけど ARM では意味を持たないと。ラベルの意味があるのでそのままにしている。