Fixes for running FreeBSD buildworld on Linux/macOS hosts

Adding an #ifdef FreeBSD to a FreeBSD-specific header may seem odd,
but these headers are used on non-FreeBSD systems during the bootstrap
tools phase.
Originally submitted downstream as https://reviews.freebsd.org/D26193

Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Alex Richardson <Alexander.Richardson@cl.cam.ac.uk>
Closes #10863