systick: Refactoring.

This change rewrites systick module a bit to use new ioreg macro, changes  `systick::setup` a bit to eliminate "boolean trap", exposes tenms() to the user, so the user can read and manipulate it.
Ahh, yes - didn't even think about it not being in an ioreg. I've opened rusy-lang/rust#16816, see how it goes.