Fix build of dtddata.S on macOS / clang

Hi! I'm fixing the build of downstream Nix on macOS, which fails on dtddata.S because of platform differences, I believe. I'm not the most familiar with Wayland or assembler specifics. For reference, here is the CI log on the Nix side: https://hydra.nixos.org/build/141980180/nixlog/2

The only thing I'm not certain about is if this is the right preprocessor check. I suppose it's actually both macOS+clang specific, and macOS+gcc or Linux+clang may again require some different syntax? (I'm not sure how much effort we want to put into things, here.)

The other thing I'm wondering is if the macro invocation shouldn't also be comma-separated for binutils (see docs), though it seems to accept both. It may not matter much for this change, because the line is changed anyway to get the correct symbol name.