Redesign the interrupt controller crate; support x86_64 (#1051)

* The `interrupt_controller` crate still is not used on x86_64, 
  but it is now nearly fully implemented.

* Move initialization of x86-specific interrupt controllers (i.e., APIC)
  into the x86-specific module of `interrupt_controller`,
  which makes more sense than doing it from the `device_manager`.
  * As such, the `captain` can now unconditionally initialize
    the interrupt controller crate, which works on all architectures.

Two remaining to-do items, which will come in a future PR:
1. Implement `enable_local_timer_interrupt()` for aarch64
   * This requires moving some aarch64-specific functionality
     for setting up the system's per-CPU timer out of the `interrupts`
     crate and into another crate.
2. Implement `SystemInterruptController::get()` for x86_64.
   * This is a bit challenging because there can be _multiple_
     system-wide interrupt controllers (I/O APICs) on x86_64,
     so we can't simply implement a basic getter function.
   * We may need to offer an iterator interface, or perhaps a
     getter that takes the current CPU or NUMA domain or
     some kind of other identifier into account.

* In the future, we'll move each `LocalInterruptController` instance
  into CPU-local storage to allow for very fast access.