Optimize bit counting using __builtin_popcount

Use the __builtin_popcount intrinsic to optimize the bit counting
function if the compiler supports it. This change replaces the manual
bit counting algorithm with the more efficient built-in function, which
leverages hardware support on compatible processors.

This modification ensures that the code remains backward-compatible by
falling back to the original implementation when __builtin_popcount is
not available.

include/afl-fuzz.h

@@ -116,6 +116,10 @@
   #include <TargetConditionals.h>
 #endif
 
+#ifndef __has_builtin
+  #define __has_builtin(x) 0
+#endif
+
 #undef LIST_FOREACH                                 /* clashes with FreeBSD */
 #include "list.h"
 #ifndef SIMPLE_FILES