add zbb check macro

snappy-stubs-internal.h

@@ -304,8 +304,8 @@ class Bits {
   void operator=(const Bits&);
 };
 
+// In RISC-V,  CLZ  is supported by instructions from the ZBB bit-manipulation extension.
 #if HAVE_BUILTIN_CTZ
-
 inline int Bits::Log2FloorNonZero(uint32_t n) {
   assert(n != 0);
   // (31 ^ x) is equivalent to (31 - x) for x in [0, 31]. An easy proof
@@ -393,6 +393,7 @@ inline int Bits::FindLSBSetNonZero(uint32_t n) {
 
 #endif  // End portable versions.
 
+// the Zbb bit-manipulation extension where ctz is guaranteed.
 #if HAVE_BUILTIN_CTZ
 
 inline int Bits::FindLSBSetNonZero64(uint64_t n) {