powerpc/64s: add do_bad_page_fault_segv handler

This function acts like an interrupt handler so it needs to follow
the standard interrupt handler function signature which will be
introduced in a future change.

Signed-off-by: Nicholas Piggin <npiggin@gmail.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20210130130852.2952424-13-npiggin@gmail.com