riscv: Schedule next hardclock tick in the future, not the past.

If we have missed hardclock ticks, schedule up to one tick interval
in the future anyway; don't try to play hardclock catchup by
scheduling for when the next hardclock tick _should_ have been, in
the past, leading to ticking as fast as possible until we've caught
up.  as fast as possible until we've caught up.

Playing hardclock catchup triggers heartbeat panics when continuing
from ddb, if you've been in ddb for >15sec.  Other hardclock drivers
like x86 lapic don't play hardclock catchup either.

PR kern/57920

(riastradh)