Force --enable-debug on FreeBSD if INVARIANTS is set

There's already logic to force INVARIANTS on for building if it's
present in the running kernel; however, not having DEBUG enabled
when DEBUG and INVARIANTS are can cause strange panics.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Ryan Moeller <ryan@iXsystems.com>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #12185
Closes #12163