selftests/bpf: Support ppc64el in vmtest

With a rootfs built using libbpf's BPF CI [1], we can run specific tests
as follows:

$ ../libbpf-ci/rootfs/mkrootfs_debian.sh --arch ppc64el --distro noble
$ PLATFORM=ppc64el CROSS_COMPILE=powerpc64le-linux-gnu- \
    tools/testing/selftests/bpf/vmtest.sh \
    -l libbpf-vmtest-rootfs-*-noble-ppc64el.tar.zst \
    -- ./test_progs -t verifier_array_access

Does not include a DENYLIST or support for KVM for now.

[1] https://github.com/libbpf/ci



Signed-off-by: Luis Gerhorst <luis.gerhorst@fau.de>
Link: https://lore.kernel.org/r/20250619140854.2135283-1-luis.gerhorst@fau.de


Signed-off-by: Alexei Starovoitov <ast@kernel.org>