cpuinfo: Added SDL_HasAVX512F().

This checks for the "foundation" AVX-512 instructions (that all AVX-512
compatible CPUs support).

include/SDL_cpuinfo.h

@@ -159,6 +159,11 @@ extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX(void);
  */
 extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX2(void);
 
+/**
+ *  This function returns true if the CPU has AVX-512F (foundation) features.
+ */
+extern DECLSPEC SDL_bool SDLCALL SDL_HasAVX512F(void);
+
 /**
  *  This function returns true if the CPU has NEON (ARM SIMD) features.
  */