intrng: allow specifying an interrupt number during allocation

[ehem_freebsd_m5p.com]

Summary

All interrupt architectures besides INTRNG expect interrupt numbers to
be selected outside. Increase INTRNG's cross-architecture potential by
allowing this approach.

[ehem_freebsd_m5p.com]

This is more in the direction of D32504. Theory is, can INTRNG be adjusted to provide sufficient functionality to replace the current x86 interrupt framework?

In this initial form I used 0x80 as this is nominally a flag meant for use by isrc_alloc_irq() and doesn't really need to be stored permanently on the intr_irqsrc structure. The value doesn't really matter since it can readily by changed via intr.h, but seemed best to somehow hint it was a setup-only flag.

Other feature needed to subsume PowerPC interrupts is to be able to hash values for the irq_sources[] table. That though brings in the issue of how best to deal with collisions?

[ehem_freebsd_m5p.com]

I now expect to abandon this. At this point I've got a PoC implementation of using a resource manager to handle the interrupt number ranges. This was a thought, but it seems unlikely now.

[ehem_freebsd_m5p.com]

Marking D32632 as abandoned. I've got something which approaches the same goal, but is distinct enough to not take over D32632.