Produce epapr images for PPC64le

Per https://gitlab.com/Linaro/tuxbuild/-/issues/83:

For PPC64le, it looks like we need Image.epapr:

$ ARCH=powerpc CROSS_COMPILE=powerpc64le-linux-gnu- make CC=clang -j pseries_defconfig

via Looks like just zImage was produced, but I think for qemu we need the zImage.epapr (compressed kernel image).