powerpc implicit FPU usage (PR847)

[Till Straumann]

I'm about to commit a few patches addressing PR847.

This will strictly disable the FPU for all non-floating
point tasks and ISRs (powerpc, 'new' exception processing).

It is likely that some people will experience
crashes because there *is* illegal FPU usage.

We might want to set CPU_ALL_TASKS_ARE_FP to TRUE
on PPCs with a hardware FPU. A more flexible approach
would be a slot in the CPU configuration table.

Comments ?

-- Till