cpuinfo: first attempt at SDL_HasNEON() implementation.

include/SDL_cpuinfo.h

@@ -144,6 +144,11 @@ extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX(void);
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX2(void);
 
+/**
+ *  This function returns true if the CPU has NEON (ARM SIMD) features.
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_HasNEON(void);
+
 /**
  *  This function returns the amount of RAM configured in the system, in MB.
  */