makesyscall: Stop generating $FreeBSD$

With 14 coming, we no longer need to generate the $FreeBSD$. We can
likely MFC that to 13 as well.

MFC After: 2 weeks
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D39879

(cherry picked from commit 61fe63f698148f8d63fe6f366c5e20bc7ad60b87)