klib,int64: implement clzsi2() and ctzsi2()

* this is to support 64-bit divisions in fceux

libs/klib/src/int64.c

@@ -283,17 +283,19 @@ __umoddi3(du_int a, du_int b)
 }
 
 
-#include <klib.h>
-
 // for more details, refer to
 // https://gcc.gnu.org/onlinedocs/gccint/Integer-library-routines.html
 
-// only use for linking
-
 int __clzsi2 (unsigned int a) {
-  _halt(1);
+  if (a == 0) return 0;
+  int bit = 31;
+  while ((a & (1 << bit)) == 0) bit --;
+  return bit;
 }
 
 int __ctzsi2 (unsigned int a) {
-  _halt(1);
+  if (a == 0) return 0;
+  int bit = 0;
+  while ((a & (1 << bit)) == 0) bit ++;
+  return bit;
 }