Timer, GIC, IRQ support for aarch64

[NathanRoyer]

This PR will help us discuss the support of these features for aarch64:

• ARMv8 Generic Timer
• ARMv8 Generic Interrupt Controller (GIC)
• the use of these devices in the context of interrupt management & handling via the interrupts crate (and maybe more crates)

[kevinaboos]

Avoid using "magic" numbers as much as possible; instead, you should define consts for things like affinity bitmaps and bitshifts.

Kindly add higher-level comments to each module and each function so we know what the module is for and what the function generally does. It's a bit hard to understand things with all of the acronyms; ideally the code (with comments) should be self-explanatory, such that the reader wouldn't have to fully know all details of the aarch64 interrupt spec to understand what the code is doing.
For example, someone reading the file gic/src/dist.rs or cpu2 or cpu3 probably wouldn't know what it's for unless they were already intimately familiar with aarch64 spec terminology.

[kevinaboos]

I think the latter half of kernel/interrupts/src/aarch64/mod.rs should be moved into an "exception"-related crate or module, since they're not strictly about interrupt handling; they're mostly early/default exception handlers.