Fix -Wundef warnings due to use of unguarded __3DS__

src/cpuinfo/SDL_cpuinfo.c

@@ -453,7 +453,7 @@ static int CPU_haveNEON(void)
     return 1; /* ARMv8 always has non-optional NEON support. */
 #elif defined(__VITA__)
     return 1;
-#elif __3DS__
+#elif defined(__3DS__)
     return 0;
 #elif defined(__APPLE__) && defined(__ARM_ARCH) && (__ARM_ARCH >= 7)
     /* (note that sysctlbyname("hw.optional.neon") doesn't work!) */

src/file/SDL_rwops.c

@@ -575,7 +575,7 @@ SDL_RWFromFile(const char *file, const char *mode)
 #elif __WINRT__
         FILE *fp = NULL;
         fopen_s(&fp, file, mode);
-#elif __3DS__
+#elif defined(__3DS__)
         FILE *fp = N3DS_FileOpen(file, mode);
 #else
         FILE *fp = fopen(file, mode);