aix: detect power10 processor.

For -mcpu=native, GCC needs to detect the processor.  This
patch adds the processor value for Power10.

	Suggested by Kevin Alder.

	* config/rs6000/driver-rs6000.c (detect_processor_aix): Add
	power10.

gcc/config/rs6000/driver-rs6000.c

@@ -418,6 +418,9 @@ detect_processor_aix (void)
     case 0x20000:
       return "power9";
 
+    case 0x40000:
+      return "power10";
+
     default:
       return "powerpc";
     }