selftests/powerpc: Properly handle failure in switch_endian_test

On older CPUs the switch_endian() syscall doesn't work. Currently that
causes the switch_endian_test to just crash. Instead detect the
failure and properly exit with a failure message.
Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
Link: https://lore.kernel.org/r/20200819015727.1977134-9-mpe@ellerman.id.au

tools/testing/selftests/powerpc/switch_endian/switch_endian_test.S

@@ -3,9 +3,13 @@
 
 	.data
 	.balign 8
-message:
+success_message:
 	.ascii "success: switch_endian_test\n\0"
 
+	.balign 8
+failure_message:
+	.ascii "failure: switch_endian_test\n\0"
+
 	.section ".toc"
 	.balign 8
 pattern:
@@ -64,6 +68,9 @@ FUNC_START(_start)
 	li r0, __NR_switch_endian
 	sc
 
+	tdi   0, 0, 0x48	// b +8 if the endian was switched
+	b     .Lfail	  	// exit if endian didn't switch
+
 #include "check-reversed.S"
 
 	/* Flip back, r0 already has the switch syscall number */
@@ -71,12 +78,20 @@ FUNC_START(_start)
 
 #include "check.S"
 
+	ld	r4, success_message@got(%r2)
+	li	r5, 28	// strlen(success_message)
+	li	r14, 0	// exit status
+.Lout:
 	li	r0, __NR_write
 	li	r3, 1	/* stdout */
-	ld	r4, message@got(%r2)
-	li	r5, 28	/* strlen(message3) */
 	sc
 	li      r0, __NR_exit
-	li	r3, 0
+	mr	r3, r14
 	sc
 	b       .
+
+.Lfail:
+	ld	r4, failure_message@got(%r2)
+	li	r5, 28	// strlen(failure_message)
+	li	r14, 1
+	b	.Lout