Fix build of aibs with base gcc on i386

Add a few intermediate casts to intptr_t to suppress "cast to pointer
from integer of different size" warnings from gcc. In this case, the
'arg2' part of SYSCTL_HANDLER_ARGS is used to pass in a pointer, via an
intermediate intmax_t, so no information is lost.

Reviewed by: avg
MFC after: 3 days
Differential Revision: https://reviews.freebsd.org/D15725