stdbit: fix CTZ fall-back

The computed value was incorrect for types smaller than long long. The
code is not really used by anything at the moment, since we pretty much
compile with Clang or GCC only.

compat/stdbit/stdbit.h

@@ -178,6 +178,8 @@ static inline unsigned int __stdc_trailing_zeros(unsigned long long value,
 {
     unsigned int zeros = size * CHAR_BIT;
 
+    values <<= (sizeof (value) * CHAR_BIT) - zeros;
+
     while (value != 0) {
         value <<= 1;
         zeros--;