make maximum interrupt number tunable on ARM, ARM64, MIPS, and RISC-V

Use a machdep.nirq tunable intead of compile-time constant NIRQ
as a value for maximum number of interrupts. It allows keep a system
footprint small by default with an option to increase the limit
for large systems like server-grade ARM64

Reviewd by: mhorne
Differential Revision: https://reviews.freebsd.org/D27844
Submitted by: Klara, Inc.
Sponsored by: Ampere Computing