Skip async_destroy_001_pos on 32-bit systems

The async_destroy_001_pos test case currently hangs when testing on
a 32-bit system. Conditionally skip this test case on 32-bit
systems until the root cause is identified and resolved.

Reviewed-by: Tom Caputi <tcaputi@datto.com>
Reviewed-by: Tim Chase <tim@chase2k.com>
Signed-off-by: Brian Behlendorf <behlendorf1@llnl.gov>
Issue #5352
Issue #5347