bpf/docs: Update list of architectures supported.

vmtest.sh also supports s390x now.

Signed-off-by: KP Singh <kpsingh@kernel.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
Link: https://lore.kernel.org/bpf/20220307133048.1287644-2-kpsingh@kernel.org