fix C implementation of a_clz_32

this broke mallocng size_to_class on archs without a native
implementation of a_clz_32. the incorrect logic seems to have been
something i derived from a related but distinct log2-type operation.
with the change made here, it passes an exhaustive test.

as this function is new and presently only used by mallocng, no other
functionality was affected.

src/internal/atomic.h

@@ -319,7 +319,7 @@ static inline int a_clz_64(uint64_t x)
 #define a_clz_32 a_clz_32
 static inline int a_clz_32(uint32_t x)
 {
-	x--;
+	x >>= 1;
 	x |= x >> 1;
 	x |= x >> 2;
 	x |= x >> 4;