zfs: Increase compatibility for different environments

libsa uses the full OpenZFS compilation environment when we build this
included in zfs.c there. Other parts of the tree have not been adapted
to the full OpenZFS environment yet and need these ASSERT* defines to
build properly. Since the ASSERT* macros are normally defined in
sys/debug.h in the OpenZFS compatibility spl, only define them when
ASSERT3S is not defined to cope with the parts of the loader that don't
yet use the full OpenZFS environment.

Sponsored by: Netflix
Reviewed by: tsoome, delphij
Differential Revision: https://reviews.freebsd.org/D35893