Fix zdb segfault

On 32-bit systems setting 'zfs_arc_max = 256M' in zdb results in the
following segmentation fault. Rather than reverting 0ec0724 which
introduced this flaw this code is only used for 64-bit builds.

Segmentation fault (core dumped)
ztest: '/sbin/zdb -bcc -d -U /var/tmp/zpool.cache ztest' exit code 139
child exited with code 3

Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>