Fix build on x86-64/darwin.
* It turns out that darwin preprocesses assembly files slightly
differently from every other platform. Possibly something related
to the rather VMSish -fdollars-in-identifiers option to gcc.
* Use parens to break the preprocessor "macro" we're trying to
use away from the dollar-sign literal-constant marker it's getting
* Reported by nikodemus in #sbcl.