selftests: x86: override clean in lib.mk to fix warnings

Add override with EXTRA_CLEAN for lib.mk clean to fix the following
warnings from clean target run.

Makefile:44: warning: overriding recipe for target 'clean'
../lib.mk:55: warning: ignoring old recipe for target 'clean'
Signed-off-by: Shuah Khan <shuahkh@osg.samsung.com>

tools/testing/selftests/x86/Makefile

@@ -40,8 +40,7 @@ all_32: $(BINARIES_32)
 
 all_64: $(BINARIES_64)
 
-clean:
-	$(RM) $(BINARIES_32) $(BINARIES_64)
+EXTRA_CLEAN := $(BINARIES_32) $(BINARIES_64)
 
 $(BINARIES_32): $(OUTPUT)/%_32: %.c
 	$(CC) -m32 -o $@ $(CFLAGS) $(EXTRA_CFLAGS) $^ -lrt -ldl -lm