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>

Makefile

@@ -1367,7 +1367,7 @@ kselftest-%: headers FORCE
 PHONY += kselftest-merge
 kselftest-merge:
 	$(if $(wildcard $(objtree)/.config),, $(error No .config exists, config your kernel first!))
-	$(Q)find $(srctree)/tools/testing/selftests -name config | \
+	$(Q)find $(srctree)/tools/testing/selftests -name config -o -name config.$(UTS_MACHINE) | \
 		xargs $(srctree)/scripts/kconfig/merge_config.sh -y -m $(objtree)/.config
 	$(Q)$(MAKE) -f $(srctree)/Makefile olddefconfig