Allow swi_sched() to be called from NMI context.

Allow swi_sched() to be called from NMI context.

For purposes of handling hardware error reported via NMIs I need a way to
escape NMI context, being too restrictive to do something significant.

To do it this change introduces new swi_sched() flag SWI_FROMNMI, making
it careful about used KPIs.  On platforms allowing IPI sending from NMI
context (x86 for now) it immediately wakes clk_intr_event via new IPI_SWI,
otherwise it works just like SWI_DELAY.  To handle the delayed SWIs this
patch calls clk_intr_event on every hardclock() tick.

MFC after:	2 weeks
Sponsored by:	iXsystems, Inc.
Differential Revision:	https://reviews.freebsd.org/D25754