Fix zdb_dump_block on little endian systems

Fix zdb_dump_block on little endian systems

When dumping a block on a little endian system the data must be
byte swapped to display correctly.  Example incorrect output:

$ echo 0123456789abcdef > aaa
$ zdb -eR pp 3:1ee00:200
3:1ee00:200
          0 1 2 3 4 5 6 7   8 9 a b c d e f  0123456789abcdef
000000:  3736353433323130  6665646362613938  0123456789abcdef
000010:  000000000000000a  0000000000000000  ................

Signed-off-by: Chunwei Chen <david.chen@osnexus.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Closes #4020