Fixed lvlhsh test on 64-bit big-endian systems.

The nxt_murmur_hash2() generated 4-byte hash that was stored in uintptr_t,
which was 8 bytes long on 64-bit systems.  At each iteration, it took the
previous key and hashed it again.

The problem was that it took only the first 4 bytes of the key, and these
4 bytes were always zero on 64-bit big-endian system.  That resulted in
equal keys at each iteration.

The bug was discovered on IBM/S390x.

src/test/nxt_lvlhsh_test.c

@@ -131,7 +131,7 @@ nxt_int_t
 nxt_lvlhsh_test(nxt_thread_t *thr, nxt_uint_t n, nxt_bool_t use_pool)
 {
     void                      *value;
-    uintptr_t                 key;
+    uint32_t                  key;
     nxt_mp_t                  *mp;
     nxt_nsec_t                start, end;
     nxt_uint_t                i;