selftests/powerpc: Always build the tm-poison test 64-bit

The tm-poison test includes inline asm which is 64-bit only, so the
test must be built 64-bit in order to work.

Otherwise it fails, eg:
  # file tm-poison
  tm-poison: ELF 32-bit MSB executable, PowerPC or cisco 4500, version 1 (SYSV) ...
  # ./tm-poison
  test: tm_poison_test
  Unknown value 0x1fff71150 leaked into f31!
  Unknown value 0x1fff710c0 leaked into vr31!
  failure: tm_poison_test

Fixes: a003365c ("powerpc/tm: Add tm-poison test")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200403095656.3772005-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/tm/Makefile

@@ -25,6 +25,7 @@ $(OUTPUT)/tm-unavailable: CFLAGS += -O0 -pthread -m64 -Wno-error=uninitialized -
 $(OUTPUT)/tm-trap: CFLAGS += -O0 -pthread -m64
 $(OUTPUT)/tm-signal-context-force-tm: CFLAGS += -pthread -m64
 $(OUTPUT)/tm-signal-pagefault: CFLAGS += -pthread -m64
+$(OUTPUT)/tm-poison: CFLAGS += -m64
 
 SIGNAL_CONTEXT_CHK_TESTS := $(patsubst %,$(OUTPUT)/%,$(SIGNAL_CONTEXT_CHK_TESTS))
 $(SIGNAL_CONTEXT_CHK_TESTS): tm-signal.S