ARM: Create configure option --enable-arm-simd to govern assembly optimizations

include/SDL_cpuinfo.h

@@ -186,6 +186,11 @@ extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX2(void);
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX512F(void);
 
+/**
+ *  This function returns true if the CPU has ARM SIMD (ARMv6) features.
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_HasARMSIMD(void);
+
 /**
  *  This function returns true if the CPU has NEON (ARM SIMD) features.
  */