[POWERPC] Minor paca optimisation

Move the slb_shadow_ptr field into the first cache line since it is
(like everything there) read-only after boot.  It is in fact statically
initialised and thereafter only read.

Signed-off-by: Stephen Rothwell <sfr@canb.auug.org.au>
Acked-by: Michael Neuling <mikey@neuling.org>
Signed-off-by: Paul Mackerras <paulus@samba.org>