add __builtin_ctz and __builtin_ctzl

Summary: Add missing WIN32 compat builtins

Reviewed By: Orvid

Differential Revision: D7276240

fbshipit-source-id: 094c0861126b3477e9112987f033edacbde4aca4

folly/portability/Builtins.h

@@ -55,6 +55,15 @@ FOLLY_ALWAYS_INLINE int __builtin_clzll(unsigned long long x) {
   return int(_BitScanReverse64(&index, x) ? 63 - index : 64);
 }
 
+FOLLY_ALWAYS_INLINE int __builtin_ctz(unsigned int x) {
+  unsigned long index;
+  return int(_BitScanForward(&index, (unsigned long)x) ? index : 32);
+}
+
+FOLLY_ALWAYS_INLINE int __builtin_ctzl(unsigned long x) {
+  return __builtin_ctz((unsigned int)x);
+}
+
 FOLLY_ALWAYS_INLINE int __builtin_ctzll(unsigned long long x) {
   unsigned long index;
   return int(_BitScanForward64(&index, x) ? index : 64);