powerpc: Implement task_pt_regs() accessor

The task_pt_regs() macro allows access to the pt_regs of a given task.

This macro is not currently defined for the powerpc architecture, but
we need it for some upcoming utrace additions.

Signed-off-by: Srinivasa DS <srinivasa@in.ibm.com>
Signed-off-by: Benjamin Herrenschmidt <benh@kernel.crashing.org>