Port PowerPC kqueue(2) compat32 fix in r320500 to MIPS.

All 32bit MIPS ABIs align uint64_t on 8-byte. Since struct kevent32
is defined using 32bit types to avoid extra alignment on amd64/i386,
layout of the structure needs paddings on PowerPC and apparently MIPS.

Reviewed by: jhb
Sponsored by: The FreeBSD Foundation
Differential revision: https://reviews.freebsd.org/D11434