Replace ISAINFO with is_32bit function

The isainfo(1) utility was used by the ZFS Test Suite to determine
when running on a 32-bit platform.  This non-portable check has been
replaced with an is_32bit helper function which uses getconf(1).
The getconf(1) utility is available for Linux, FreeBSD, and Illumos.

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

tests/zfs-tests/include/commands.cfg.in

@@ -48,7 +48,6 @@ export HEAD="@HEAD@"
 export HOSTNAME="@HOSTNAME@"
 export ID="@ID@"
 export IOSTAT="@IOSTAT@"
-export ISAINFO="@ISAINFO@"
 export KILL="@KILL@"
 export KSH="@KSH@"
 export KSTAT="@KSTAT@"