build system: disable 3dNow! support by default

Closes: https://github.com/libsdl-org/SDL/issues/11472.
(cherry picked from commit 51570d9257)

include/SDL_cpuinfo.h

@@ -53,9 +53,11 @@ _m_prefetch(void *__P)
 #ifndef __MMX__
 #define __MMX__
 #endif
+/*
 #ifndef __3dNOW__
 #define __3dNOW__
 #endif
+*/
 #endif
 #ifndef __SSE__
 #define __SSE__