tests: Fix up docker cross builds for ppc64 (BE) targets

We currently have docker cross building targets for powerpc (32-bit, BE)
and ppc64el (64-bit, LE), but not for pcp64 (64-bit, BE).  This is an
irritating gap in make check-tcg coverage so correct it.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
3 files changed