powerpc: Explicitly copy elements of pt_regs

Gcc 4.3 produced this warning:

arch/powerpc/kernel/signal_64.c: In function 'restore_sigcontext':
arch/powerpc/kernel/signal_64.c:161: warning: array subscript is above array bounds

This is caused by us copying to aliases of elements of the pt_regs
structure.  Make those explicit.

This adds one extra __get_user and unrolls a loop.

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