freebsd32: Fix layout of struct shmid_kernel32.

The kernel pointers in this structure need to be 32-bit pointers,
not native pointers to 32-bit integers.

Reviewed by: kib
Sponsored by: The University of Cambridge, Google Inc.
Differential Revision: https://reviews.freebsd.org/D33905

(cherry picked from commit da7fc5c33f9a4c906068a9a43f43f8d295100418)