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/pitch_sse2.c

@@ -29,6 +29,7 @@
 #include "config.h"
 #endif
 
+#if defined (__i386__) || defined(__x86_64__)
 #include <xmmintrin.h>
 #include <emmintrin.h>
 
@@ -93,3 +94,4 @@ opus_val32 celt_inner_prod_sse2(const opus_val16 *x, const opus_val16 *y,
     return sum;
 }
 #endif
+#endif