bpf/docs: Update list of architectures supported.

vmtest.sh also supports s390x now.

Signed-off-by: KP Singh <[e-mail]>
Signed-off-by: Andrii Nakryiko <[e-mail]>
Link: https://lore.kernel.org/bpf/[e-mail]

tools/testing/selftests/bpf/README.rst

···
           If you want to change pahole and llvm, you can change `PATH` environment
           variable in the beginning of script.
 
-.. note:: The script currently only supports x86_64.
+.. note:: The script currently only supports x86_64 and s390x architectures.
 
 Additional information about selftest failures are
 documented here.