Split GIC in SMP examples (in #127)

Rather than:

static GLOBAL_GIC: Mutex<RefCell<Option<arm_gic::gicv3::GicV3<'static>>>> = Mutex::new(RefCell::new(None));

We should have

static GLOBAL_GIC_DIST: Mutex<RefCell<Option<arm_gic::gicv3::GicDistributor<'static>>>> = Mutex::new(RefCell::new(None));

pub unsafe fn get_redist() -> arm_gic::gicv3::GicRedistributor<'static> {
   // Read MPIDR register, and return the GIC Redistributor object for that particular core
}

That is, we have one global, shared, Mutex-projected object for the Distributor, and local per-core objects for the Redistributor.

See https://documentation-service.arm.com/static/6273be2d7e121f01fd22f145:

Start in the SMP examples and locate the GLOBAL_GIC declaration and uses of arm_gic::gicv3::GicV3. Read the GICv3 distributor and redistributor APIs, then inspect the MPIDR register access needed by get_redist. Done means the distributor is globally shared while each core obtains its own redistributor object.