Swap in % for @ to fix assembly on (at least) ARM.

Quoting from the gas manual:

> Note on targets where the @ character is the start of a comment
> (eg ARM) then another character is used instead. For example the
> ARM port uses the % character.

Luckily, % works fine on x86 too, so let's do that unconditionally.

Signed-off-by: Adam Conrad <adconrad@0c3.net>