powerpc: Clean ifdef usage in copy_thread()

Currently, a single ifdef covers SLB related bits and more generic ppc64
related bits, split this in two separate ifdef's since 64-bit BookE will
need one but not the other.

Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>
Acked-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>