UBUNTU: [Config] ppc64el: build for Power8 not Power7

UBUNTU: [Config] ppc64el: build for Power8 not Power7

BugLink: https://bugs.launchpad.net/bugs/1699627
Switch from building ppc64el with CPU_POWER7 to CPU_POWER8.
This enables HAVE_EFFICIENT_UNALIGEND_ACCESS, which:
 - makes some things faster
 - allows more eBPF programs (e.g. for XDP) to validate

CC: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Daniel Axtens <daniel.axtens@canonical.com>
Acked-by: Stefan Bader <stefan.bader@canonical.com>
Acked-by: Colin Ian King <colin.king@canonical.com>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>