linux: Provide dummy seccomp(2)

Don't emit messages; this isn't any different from a Linux kernel
built without OPTIONS_SECCOMP, so the userspace already needs to know
how to deal with it. This is also similar with how we handle seccomp
in linux_prctl().

Sponsored By: EPSRC
Differential Revision: https://reviews.freebsd.org/D33808

(cherry picked from commit 99454d3e98ff205881aa7abef39234c9afc7600c)