arch: Make a base class for Interrupts.

arch: Make a base class for Interrupts.

That abstracts the ISA further from the CPU, getting us a small step
closer to being able to build in more than one ISA at a time.

Change-Id: Ibf7e26a3df411ffe994ac1e11d2a53b656863223
Reviewed-on: https://gem5-review.googlesource.com/c/public/gem5/+/20831
Tested-by: kokoro <noreply+kokoro@google.com>
Reviewed-by: Jason Lowe-Power <jason@lowepower.com>
Reviewed-by: Andreas Sandberg <andreas.sandberg@arm.com>
Maintainer: Andreas Sandberg <andreas.sandberg@arm.com>