sys: Add a deprecation warning for 32-bit kernels.

Per recent discussions on arch@ and at the BSDCan developer summit, we
are considering removing support for 32-bit platforms (in some form)
for 15.0 (at the earliest). A final decision on what will ship in
15.0 will be made closer to the release of 15.0. However, we should
communicate the potential deprecation in 14.0 to provide notice to
users.

This commit adds a warning during boot on 32-bit kernels that they are
deprecated and may be removed in 15.0. More details will be included
in a followup commit to RELNOTES.

Reviewed by: brooks, imp, emaste
Differential Revision: https://reviews.freebsd.org/D41163