selftests: add EXTRA_CLEAN for clean target

Some testcases need the clean extra data after running. This patch
introduce the "EXTRA_CLEAN" variable to address this requirement.

After KBUILD_OUTPUT is enabled in later patch, it will be easy to
decide to if we need do the cleanup in the KBUILD_OUTPUT path(if the
testcase ran immediately after compiled).

Signed-off-by: Bamvor Jian Zhang <bamvor.zhangjian@linaro.org>
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/powerpc/switch_endian/Makefile

@@ -2,6 +2,8 @@ TEST_GEN_PROGS := switch_endian_test
 
 ASFLAGS += -O2 -Wall -g -nostdlib -m64
 
+EXTRA_CLEAN = *.o check-reversed.S
+
 include ../../lib.mk
 
 switch_endian_test: check-reversed.S
@@ -11,6 +13,3 @@ check-reversed.o: check.o
 
 check-reversed.S: check-reversed.o
 	hexdump -v -e '/1 ".byte 0x%02X\n"' $< > $@
-
-clean:
-	$(RM) $(TEST_GEN_PROGS) *.o check-reversed.S