Add De Bruijn–based ctz helper

This commit introduces a De Bruijn–based count trailing zeros (ctz)
helper. It provides a constant-time way to obtain the index of the
lowest set bit, and can be used in future bitmap priority lookup or
other bitmask-related operations.

Author: vicLin8712

include/lib/libc.h

@@ -153,3 +153,23 @@ char *getline(char *s);
 int32_t printf(const char *fmt, ...);
 int32_t snprintf(char *str, size_t size, const char *fmt, ...);
 int vsnprintf(char *str, size_t size, const char *fmt, va_list args);
+
+/* De-Bruijn-based count trailing zeros */
+inline int ctz(uint32_t v)
+{
+    /* v = 0, invalid input */
+    if (v == 0)
+        return -1;
+
+    /* De-Bruijn LUT */
+    static const uint8_t debruijn_lut[32] = {
+        0,  1,  28, 2,  29, 14, 24, 3, 30, 22, 20, 15, 25, 17, 4,  8,
+        31, 27, 13, 23, 21, 19, 16, 7, 26, 12, 18, 6,  11, 5,  10, 9};
+
+    /* Isolate rightmost bit */
+    uint32_t isolated = v & (-v);
+
+    uint32_t hash = (isolated * 0x077CB531U) >> 27;
+
+    return debruijn_lut[hash & 0x1F];
+}