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

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.