selftests/powerpc: Fix wild_bctr test to work on ppc64

The selftest I recently added to test branching to an out-of-bounds
NIP doesn't work on 64-bit big endian. It does fail but not in the
right way. That is it SEGVs trying to load from the opd at BAD_NIP,
but it never gets as far as branching to BAD_NIP.

To fix it we need to create an opd which is reachable but which holds
the bad address.

Fixes: b7683fc ("selftests/powerpc: Add a test of wild bctr")
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>

tools/testing/selftests/powerpc/mm/wild_bctr.c

@@ -105,6 +105,20 @@ static void dump_regs(void)
 	}
 }
 
+#ifdef _CALL_AIXDESC
+struct opd {
+	unsigned long ip;
+	unsigned long toc;
+	unsigned long env;
+};
+static struct opd bad_opd = {
+	.ip = BAD_NIP,
+};
+#define BAD_FUNC (&bad_opd)
+#else
+#define BAD_FUNC BAD_NIP
+#endif
+
 int test_wild_bctr(void)
 {
 	int (*func_ptr)(void);
@@ -133,7 +147,7 @@ int test_wild_bctr(void)
 
 		poison_regs();
 
-		func_ptr = (int (*)(void))BAD_NIP;
+		func_ptr = (int (*)(void))BAD_FUNC;
 		func_ptr();
 
 		FAIL_IF(1); /* we didn't segv? */