Create dummy signal queueing function so we can start to integrate other
architectures (at the cost of signals remaining broken) to tame the
dependency graph a bit and to bring in signals in a more controlled
fashion.  Log unimplemented events to it in the mean time.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Kyle Evans <kevans@FreeBSD.org>