Fix compiling on FreeBSD + gcc - don't assume illmnos bits

This looks like it was once from the illumnos compat code.
FreeBSD doesn't have cmn_err as a compiler format attribute, so
it definitely errors out.

It doesn't show up on LLVM because it doesn't trigger at all.

Add in the format flags but keep them behind #if 0 for now;
there are too many format issues that trigger when one does
format checking in the shared code.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: adrian chadd <adrian@freebsd.org>
Closes #11068
Closes #11069