powerpc/6xx: Don't use SPRN_SPRG2 for storing stack pointer while in RTAS

When calling RTAS, the stack pointer is stored in SPRN_SPRG2
in order to be able to restore it in case of machine check in RTAS.

As machine check is not a perfomance critical path, this patch
frees SPRN_SPRG2 by using a field in thread struct instead.

Signed-off-by: Christophe Leroy <christophe.leroy@c-s.fr>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>