powerpc/kretprobes: Override default function entry offset

With ABIv2, we offset 8 bytes into a function to get at the local entry
point.

mpe: NB this function is currently not called, the change to generic code to
call it is being merged via the tip tree.

Acked-by: Ananth N Mavinakayanahalli <ananth@linux.vnet.ibm.com>
Acked-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Naveen N. Rao <naveen.n.rao@linux.vnet.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>