Start to implement the force_sig_fault code. This currently just calls
queue_signal(). The bsd-user fork version of that will handle this the
synchronous nature of this call. Add signal-common.h to hold signal
helper functions like force_sig_fault.

Signed-off-by: Stacey Son <sson@FreeBSD.org>
Signed-off-by: Kyle Evans <kevans@freebsd.org>
Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>