kbuild: Merge per-arch config for kselftest-merge target

Some kselftests has a per-arch config,
e.g. tools/testing/selftests/bpf/config.s390x.

Make sure these configs are picked up by the kselftest-merge target.

Signed-off-by: Björn Töpel <bjorn@rivosinc.com>
Signed-off-by: Shuah Khan <skhan@linuxfoundation.org>