selftests/powerpc: Skip tm-signal-sigreturn-nt if TM not available

On systems where TM (Transactional Memory) is disabled the
tm-signal-sigreturn-nt test causes a SIGILL:

  test: tm_signal_sigreturn_nt
  tags: git_version:7c202575
  !! child died by signal 4
  failure: tm_signal_sigreturn_nt

We should skip the test if TM is not available.

Fixes: 34642d70 ("selftests/powerpc: Add checks for transactional sigreturn")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20191104233524.24348-1-mpe@ellerman.id.au

tools/testing/selftests/powerpc/tm/tm-signal-sigreturn-nt.c

@@ -10,10 +10,12 @@
  */
 
 #define _GNU_SOURCE
+#include <stdio.h>
 #include <stdlib.h>
 #include <signal.h>
 
 #include "utils.h"
+#include "tm.h"
 
 void trap_signal_handler(int signo, siginfo_t *si, void *uc)
 {
@@ -29,6 +31,8 @@ int tm_signal_sigreturn_nt(void)
 {
 	struct sigaction trap_sa;
 
+	SKIP_IF(!have_htm());
+
 	trap_sa.sa_flags = SA_SIGINFO;
 	trap_sa.sa_sigaction = trap_signal_handler;