Collapse interrupt thread priorities.

Allow high priority hardware interrupts to run at PI_REALTIME via
INTR_TYPE_CLK, but collapse all other hardware interrupt threads to
the next priority level (PI_INTR). Collapse all SWI priorities to
the same priority level (PI_SOFT) just below PI_INTR.

Reviewed by: kib, markj
Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D35646