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

tools/testing/selftests/bpf/README.rst

@@ -44,7 +44,7 @@ In case of linker errors when running selftests, try using static linking:
           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.