selftests/powerpc: Use CLEAN macro to fix make warning

The CLEAN macro was added in 337f1e36 to prevent the

    Makefile:50: warning: overriding recipe for target 'clean'
    ../../lib.mk:124: warning: ignoring old recipe for target 'clean'

style warnings. Expand it's use to fix another case of redefining a
target directly.
Signed-off-by: Benjamin Gray <bgray@linux.ibm.com>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://msgid.link/20230228000709.124727-2-bgray@linux.ibm.com

tools/testing/selftests/powerpc/pmu/Makefile

@@ -46,11 +46,14 @@ override define INSTALL_RULE
 	TARGET=event_code_tests; BUILD_TARGET=$$OUTPUT/$$TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET install
 endef
 
-clean:
+DEFAULT_CLEAN := $(CLEAN)
+override define CLEAN
+	$(DEFAULT_CLEAN)
 	$(RM) $(TEST_GEN_PROGS) $(OUTPUT)/loop.o
 	TARGET=ebb; BUILD_TARGET=$$OUTPUT/$$TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean
 	TARGET=sampling_tests; BUILD_TARGET=$$OUTPUT/$$TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean
 	TARGET=event_code_tests; BUILD_TARGET=$$OUTPUT/$$TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -C $$TARGET clean
+endef
 
 ebb:
 	TARGET=$@; BUILD_TARGET=$$OUTPUT/$$TARGET; mkdir -p $$BUILD_TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -k -C $$TARGET all
@@ -61,4 +64,4 @@ sampling_tests:
 event_code_tests:
 	TARGET=$@; BUILD_TARGET=$$OUTPUT/$$TARGET; mkdir -p $$BUILD_TARGET; $(MAKE) OUTPUT=$$BUILD_TARGET -k -C $$TARGET all
 
-.PHONY: all run_tests clean ebb sampling_tests event_code_tests
+.PHONY: all run_tests ebb sampling_tests event_code_tests