libzpool: set_global_var: fix endianness handling (fixes zdb -o )

Without this patch I get the error

Setting global variables is only supported on little-endian systems

when using zdb -o on my amd64 machine.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Pavel Zakharov <pavel.zakharov@delphix.com>
Signed-off-by: Christian Schwarz <me@cschwarz.com>
Closes #11602