rs6000: fix power10_hw test

The code snippet for this test was returning 1 if power10
instructions executed correctly. It should return 0 if the
test passes.

	2020-07-07  Aaron Sawdey  <acsawdey@linux.ibm.com>

gcc/testsuite/
	* lib/target-supports.exp (check_power10_hw_available):
	Return 0 for passing test.

(cherry picked from commit 7126583af5d29235584b51b3b05eeaba2adef024)

gcc/testsuite/lib/target-supports.exp

@@ -2227,7 +2227,9 @@ proc check_power10_hw_available { } {
 		/* Set e first and use +r to check if pli actually works.  */
 		long e = -1;
 		asm ("pli %0,%1" : "+r" (e) : "n" (0x12345));
-		return (e == 0x12345);
+		if (e == 0x12345)
+		  return 0;
+		return 1;
 	    }
 	} "-mcpu=power10"
     }]