selftests/powerpc/signal: Fix out-of-tree build

We should use TEST_GEN_PROGS, not TEST_PROGS. That tells the selftests
makefile (lib.mk) that those tests are generated (built), and so it
adds the $(OUTPUT) prefix for us, making the out-of-tree build work
correctly.

It also means we don't need our own clean rule, lib.mk does it.

We also have to update the signal_tm rule to use $(OUTPUT).
Signed-off-by: Joel Stanley <joel@jms.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/signal/Makefile

 # SPDX-License-Identifier: GPL-2.0
-TEST_PROGS := signal signal_tm
-
-all: $(TEST_PROGS)
-
-$(TEST_PROGS): ../harness.c ../utils.c signal.S
+TEST_GEN_PROGS := signal signal_tm
 
 CFLAGS += -maltivec
-signal_tm: CFLAGS += -mhtm
+$(OUTPUT)/signal_tm: CFLAGS += -mhtm
 
 top_srcdir = ../../../../..
 include ../../lib.mk
 
-clean:
-	rm -f $(TEST_PROGS) *.o
+$(TEST_GEN_PROGS): ../harness.c ../utils.c signal.S