kern: Mark some functions __diagused to pacify clang.

These functions are called only in KASSERT.  This shouldn't be a
problem since the recent change to make KASSERT compile (but not run)
the expression, via sizeof, so the functions are referenced -- which
I did in order to avoid having #ifdefs and regular build breakage
because someone forgot to build with or without DIAGNOSTIC.  But
apparently clang decided to make it a problem.

Maybe we should just set -Wno-unneeded-internal-declaration -- not
clear it flags any real problems, but it takes effort to research
because apparently clang has no documentation about what else it does
or why anyone decided this is objectionable enough to turn it on by
default.  (The only documentation it seems to have, two example
warning messages with no elaboration at
<https://releases.llvm.org/13.0.0/tools/clang/docs/DiagnosticsReference.html#wunneeded-internal-declaration>,
seems to indicate it is mixed up with warning about static non-inline
functions in header files.)

(riastradh)