Stop using the rid as an index in the arm timer

The order of the interrupt array doesn't matter. Store the described
interrupts at the start of the array to simplify iterating over them.

Reviewed by: imp, kevans
Sponsored by: Arm Ltd
Differential Revision: https://reviews.freebsd.org/D39094

(cherry picked from commit 5c4bd8756fbc23a48ddd8976df0b8429311fba83)