zfs: Increase compatibility for different environments

[imp]

Summary

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 the
guard for it (_SPL_DEBUG_H) is not defined. This should cover the
non-libsa boot loaders (gptzfsboot in particular) that need to use
zfssubr.c.

Sponsored by: Netflix

[tsoome]

I am wondering, if we should use #ifndef ASSERT etc instead.

[imp]

Move to using #ifndef ASSERT3S
Commit message updated locally.