C runtime: add kernel version guards on exterrctl

This allows userspace to run on a (somewhat) out of date kernel.

Avoid a __FreeBSD_version bump and use the bump from a02180cf60a6 which
has occured since exterrctl was added.

Reviewed by: kevans
Fixes: b9c8a07d4dd9 ("C runtime: enable extended error reporting from kernel")
Sponsored by: DARPA, AFRL
Differential Revision: https://reviews.freebsd.org/D50687