compat: Fix the fallback definition of stdc_trailing_zeros

While shifting "value" to left, we would iterate through all bits
of an unsigned long long, while we only expect to count through
"size * CHAR_BIT" bits; instead shift bits to the right and just
count the trailing zeros.

This fixes fate with MSVC.

Signed-off-by: Martin Storsjö <martin@martin.st>

compat/stdbit/stdbit.h

@@ -178,11 +178,14 @@ static inline unsigned int stdc_trailing_zeros_uc(unsigned char value)
 static inline unsigned int __stdc_trailing_zeros(unsigned long long value,
                                                  unsigned int size)
 {
-    unsigned int zeros = size * CHAR_BIT;
+    unsigned int zeros = 0;
 
-    while (value != 0) {
-        value <<= 1;
-        zeros--;
+    if (!value)
+        return size * CHAR_BIT;
+
+    while ((value & 1) == 0) {
+        value >>= 1;
+        zeros++;
     }
 
     return zeros;