Upgrades to Xcode project and Apple Silicon support

- Reorganized Xcode project
- Added missionpack support to Xcode project
- Incorporated changes from MaddTheSane for Apple Silicon support https://github.com/maddthesane/ioq3
- Built SDL 2.0.14 for Apple Siicon, added to existing SDL dylib
- Built SDLMain for Apple Silicon, added to existing libSDL2main.a

code/opus-1.2.1/celt/x86/celt_lpc_sse.c

@@ -29,6 +29,8 @@
 #include "config.h"
 #endif
 
+#if defined (__i386__) || defined(__x86_64__)
+
 #include <xmmintrin.h>
 #include <emmintrin.h>
 #include <smmintrin.h>
@@ -87,3 +89,4 @@ void celt_fir_sse4_1(const opus_val16 *x,
 }
 
 #endif
+#endif