amd64/libtcod.dylib: fix SDL install name

SDL was incorrectly referred to via the install name `libSDL.dylib`, which caused it to be looked up in the default path, ignoring the bundled libSDL in the same directory. The name has been changed to `@loader_path/libSDL.dylib`, which will pick up the bundled copy. (This matches the install name for the PowerPC and i386 dylibs.)