add fallback a_clz_32 implementation

some archs already have a_clz_32, used to provide a_ctz_32, but it
hasn't been mandatory because it's not used anywhere yet. mallocng
will need it, however, so add it now. it should probably be optimized
better, but doesn't seem to make a difference at present.

src/internal/atomic.h

@@ -315,4 +315,19 @@ static inline int a_clz_64(uint64_t x)
 }
 #endif
 
+#ifndef a_clz_32
+#define a_clz_32 a_clz_32
+static inline int a_clz_32(uint32_t x)
+{
+	x--;
+	x |= x >> 1;
+	x |= x >> 2;
+	x |= x >> 4;
+	x |= x >> 8;
+	x |= x >> 16;
+	x++;
+	return 31-a_ctz_32(x);
+}
+#endif
+
 #endif