Fix PIT Interrupt Not Context Switching [irq]

Created by: InsidiousMind

PIT interrupt should context switch or else all of Redox crashes. This will fix programs like the Snake game crashing all of Redox. A global AtomicUSize counter was added, and a line to switch contexts on every 10 PIT interrupts in irq.rs.

This fixes https://github.com/redox-os/redox/issues/781 (Snake Game crashes Redox #781) (Tested)