loader: Retire CTASSERT

The project is moving away from CTASSERT in favor of
_Static_assert. Cleanup the few instances in the loader proactively.

Sponsored by: Netflix
Reviewed by: manu, tsoome
Differential Revision: https://reviews.freebsd.org/D44006

(cherry picked from commit 32568e5f2435a2539b0c4177a6fefe7a9e0a0c89)