amd64: Only build aout.ko when COMPAT_FREEBSD32 is enabled

a.out.ko is useless without COMPAT_FREEBSD32, and since vdso support was
added, it requires a header built during depend phase, from the COMPAT
option.

Reported by: glebius
Reviewed by: emaste, markj
Sponsored by: The FreeBSD Foundation
MFC after: 1 week
Differential revision: https://reviews.freebsd.org/D33311