selftests/powerpc: Fix wild_bctr test to work on ppc64

The selftest I recently added to test branching to an out-of-bounds
NIP doesn't work on 64-bit big endian. It does fail but not in the
right way. That is it SEGVs trying to load from the opd at BAD_NIP,
but it never gets as far as branching to BAD_NIP.

To fix it we need to create an opd which is reachable but which holds
the bad address.

Fixes: b7683fc6 ("selftests/powerpc: Add a test of wild bctr")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>