Where possible use compiler builtins for CountBits and FindFirstBit.

src/core/bitmath_func.cpp

@@ -14,6 +14,8 @@
 
 #include "../safeguards.h"
 
+#ifndef WITH_BITMATH_BUILTINS
+
 const uint8 _ffb_64[64] = {
 	0,  0,  1,  0,  2,  0,  1,  0,
 	3,  0,  1,  0,  2,  0,  1,  0,
@@ -53,6 +55,8 @@ uint8 FindFirstBit(uint32 x)
 	return pos;
 }
 
+#endif
+
 /**
  * Search the last set bit in a 64 bit variable.
  *