kern_thread: Fix i386 p_emuldata KBI assertion

On amd64 there was 4 bytes of padding between the 20-byte p_comm and
(for LP64) 8-byte p_sysent, so the addition of p_execblock just caused
that padding to be eaten up. However, on i386, there was no such
padding, and so the addition of p_execblock rippled through to
p_emuldata.

Fixes: e1a84b7708c2 ("execve_block(): a mechanism for mutual exclusion with execve() on the process")