devel/sdl20: fix armv7-related check in SDL_atomic.h

Fixes build errors like:
In file included from /usr/ports/games/openrct2/work/OpenRCT2-0.4.1/src/openrct2-ui/CursorRepository.h:12: In file included from /usr/local/include/SDL2/SDL.h:35: /usr/local/include/SDL2/SDL_atomic.h:243:28: error: 'ARM_ARCH' is not defined, evaluates to 0 [-Werror,-Wundef] #elif (defined(arm) && ARM_ARCH >= 7) || defined(aarch64)