Store a 32-bit PT_LWPINFO struct for 32-bit process core dumps.

Process core notes for a 32-bit process running on a 64-bit host need to
use 32-bit structures so that the note layout matches the layout of notes
of a core dump of a 32-bit process under a 32-bit kernel.

Reviewed by: kib
Differential Revision: https://reviews.freebsd.org/D11407