powerpc: Merge thread_info.h

[Paul Mackerras]

David Howells writes:

> What I did is to use a general register to hold current whilst inside the
> kernel:

PPC32 already stores current in r2.  PPC64 stores a pointer to the
paca (basically a per-cpu data area) in r13 and stores current in
there.  I'm not convinced that having current in a register, rather
than being able to get it with 1 instruction, would make a significant
different, but it may be worth the experiment.

Paul.