selftests/powerpc: Abort load_unaligned_zeropad on unhandled SEGV

If the load unaligned zeropad test takes a SEGV which can't be handled,
we increment segv_error, print the offending NIP and then return without
taking any further action. In almost all cases this means we'll just
take the SEGV again, and loop eternally spamming the console.

Instead just abort(), it's a fatal error in the test. The test harness
will notice that the child died and print a nice message for us.

Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>