[RS6000] Built-in __PCREL__ define

Useful in assembly to know details of power10 function calls.

	* config/rs6000/rs6000-c.c (rs6000_target_modify_macros):
	Conditionally define __PCREL__.

gcc/config/rs6000/rs6000-c.c

@@ -597,6 +597,9 @@ rs6000_target_modify_macros (bool define_p, HOST_WIDE_INT flags,
   /* Tell the user if we support the MMA instructions.  */
   if ((flags & OPTION_MASK_MMA) != 0)
     rs6000_define_or_undefine_macro (define_p, "__MMA__");
+  /* Whether pc-relative code is being generated.  */
+  if ((flags & OPTION_MASK_PCREL) != 0)
+    rs6000_define_or_undefine_macro (define_p, "__PCREL__");
 }
 
 void