Override timer interrupt

[Rune Torgersen]

> From: Benjamin Herrenschmidt> 
> > The TB register is only ued for offsets from the last 
> jiffie, not as a
> > continous offset, so then it works out pretty good.
>
> The date is derived from the absolute TB value though...

Huh? Not in 2.6.18/arch/ppc at least, unless I'm completely
misundrstanding the code.
ppc/powerpc seems to be using clocksource_jiffies as the clock source,
and teh gettimeofday gets xtime + a offset from last jiffie, and xtime
is updated with a fixed amount per tick.

Things have changed a lot since I last delved deep into this to try to
ge an accurate freerunning clock (2.6.12).